wifi: mt76: do not run mt76u_status_worker if the device is not running

Fix the following NULL pointer dereference avoiding to run
mt76u_status_worker thread if the device is not running yet.

KASAN: null-ptr-deref in range
[0x0000000000000000-0x0000000000000007]
CPU: 0 PID: 98 Comm: kworker/u2:2 Not tainted 5.14.0+ #78 Hardware
name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
Workqueue: mt76 mt76u_tx_status_data
RIP: 0010:mt76x02_mac_fill_tx_status.isra.0+0x82c/0x9e0
Code: c5 48 b8 00 00 00 00 00 fc ff df 80 3c 02 00 0f 85 94 01 00 00
48 b8 00 00 00 00 00 fc ff df 4d 8b 34 24 4c 89 f2 48 c1 ea 03 <0f>
b6
04 02 84 c0 74 08 3c 03 0f 8e 89 01 00 00 41 8b 16 41 0f b7
RSP: 0018:ffffc900005af988 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffffc900005afae8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff832fc661 RDI: ffffc900005afc2a
RBP: ffffc900005afae0 R08: 0000000000000001 R09: fffff520000b5f3c
R10: 0000000000000003 R11: fffff520000b5f3b R12: ffff88810b6132d8
R13: 000000000000ffff R14: 0000000000000000 R15: ffffc900005afc28
FS:  0000000000000000(0000) GS:ffff88811aa00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa0eda6a000 CR3: 0000000118f17000 CR4: 0000000000750ef0
PKRU: 55555554
Call Trace:
 mt76x02_send_tx_status+0x1d2/0xeb0
 mt76x02_tx_status_data+0x8e/0xd0
 mt76u_tx_status_data+0xe1/0x240
 process_one_work+0x92b/0x1460
 worker_thread+0x95/0xe00
 kthread+0x3a1/0x480
 ret_from_fork+0x1f/0x30
Modules linked in:
--[ end trace 8df5d20fc5040f65 ]--
RIP: 0010:mt76x02_mac_fill_tx_status.isra.0+0x82c/0x9e0
Code: c5 48 b8 00 00 00 00 00 fc ff df 80 3c 02 00 0f 85 94 01 00 00
48 b8 00 00 00 00 00 fc ff df 4d 8b 34 24 4c 89 f2 48 c1 ea 03 <0f>
b6
04 02 84 c0 74 08 3c 03 0f 8e 89 01 00 00 41 8b 16 41 0f b7
RSP: 0018:ffffc900005af988 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffffc900005afae8 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff832fc661 RDI: ffffc900005afc2a
RBP: ffffc900005afae0 R08: 0000000000000001 R09: fffff520000b5f3c
R10: 0000000000000003 R11: fffff520000b5f3b R12: ffff88810b6132d8
R13: 000000000000ffff R14: 0000000000000000 R15: ffffc900005afc28
FS:  0000000000000000(0000) GS:ffff88811aa00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fa0eda6a000 CR3: 0000000118f17000 CR4: 0000000000750ef0
PKRU: 55555554

Moreover move stat_work schedule out of the for loop.
Reported-by: Dokyung Song <dokyungs@yonsei.ac.kr>
Co-developed-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

drivers/net/wireless/mediatek/mt76/usb.c

@@ -766,6 +766,9 @@ static void mt76u_status_worker(struct mt76_worker *w)
 	struct mt76_queue *q;
 	int i;
 
+	if (!test_bit(MT76_STATE_RUNNING, &dev->phy.state))
+		return;
+
 	for (i = 0; i < IEEE80211_NUM_ACS; i++) {
 		q = dev->phy.q_tx[i];
 		if (!q)
@@ -785,11 +788,11 @@ static void mt76u_status_worker(struct mt76_worker *w)
 			wake_up(&dev->tx_wait);
 
 		mt76_worker_schedule(&dev->tx_worker);
+	}
 
 	if (dev->drv->tx_status_data &&
 	    !test_and_set_bit(MT76_READING_STATS, &dev->phy.state))
 		queue_work(dev->wq, &dev->usb.stat_work);
-	}
 }
 
 static void mt76u_tx_status_data(struct work_struct *work)