net: dsa: mv88e6060: prevent crash on an unused port

If the port isn't a CPU port nor a user port, 'cpu_dp'
is a null pointer and a crash happened on dereferencing
it in mv88e6060_setup_port():

[    9.575872] Unable to handle kernel NULL pointer dereference at virtual address 00000014
...
[    9.942216]  mv88e6060_setup from dsa_register_switch+0x814/0xe84
[    9.948616]  dsa_register_switch from mdio_probe+0x2c/0x54
[    9.954433]  mdio_probe from really_probe.part.0+0x98/0x2a0
[    9.960375]  really_probe.part.0 from driver_probe_device+0x30/0x10c
[    9.967029]  driver_probe_device from __device_attach_driver+0xb8/0x13c
[    9.973946]  __device_attach_driver from bus_for_each_drv+0x90/0xe0
[    9.980509]  bus_for_each_drv from __device_attach+0x110/0x184
[    9.986632]  __device_attach from bus_probe_device+0x8c/0x94
[    9.992577]  bus_probe_device from deferred_probe_work_func+0x78/0xa8
[    9.999311]  deferred_probe_work_func from process_one_work+0x290/0x73c
[   10.006292]  process_one_work from worker_thread+0x30/0x4b8
[   10.012155]  worker_thread from kthread+0xd4/0x10c
[   10.017238]  kthread from ret_from_fork+0x14/0x3c

Fixes: 0abfd494 ("net: dsa: use dedicated CPU port")
CC: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
CC: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Sergei Antonov <saproj@gmail.com>
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220811070939.1717146-1-saproj@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/mv88e6060.c

@@ -118,6 +118,9 @@ static int mv88e6060_setup_port(struct mv88e6060_priv *priv, int p)
 	int addr = REG_PORT(p);
 	int ret;
 
+	if (dsa_is_unused_port(priv->ds, p))
+		return 0;
+
 	/* Do not force flow control, disable Ingress and Egress
 	 * Header tagging, disable VLAN tunneling, and set the port
 	 * state to Forwarding.  Additionally, if this is the CPU