rtlwifi: fix memory leak in rtl92c_set_fw_rsvdpagepkt()

This leak was found by testing the EDIMAX EW-7612 on Raspberry Pi 3B+ with
Linux 5.4-rc5 (multi_v7_defconfig + rtlwifi + kmemleak) and noticed a
single memory leak during probe:

unreferenced object 0xec13ee40 (size 176):
  comm "kworker/u8:1", pid 36, jiffies 4294939321 (age 5580.790s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<fc1bbb3e>] __netdev_alloc_skb+0x9c/0x164
    [<863dfa6e>] rtl92c_set_fw_rsvdpagepkt+0x254/0x340 [rtl8192c_common]
    [<9572be0d>] rtl92cu_set_hw_reg+0xf48/0xfa4 [rtl8192cu]
    [<116df4d8>] rtl_op_bss_info_changed+0x234/0x96c [rtlwifi]
    [<8933575f>] ieee80211_bss_info_change_notify+0xb8/0x264 [mac80211]
    [<d4061e86>] ieee80211_assoc_success+0x934/0x1798 [mac80211]
    [<e55adb56>] ieee80211_rx_mgmt_assoc_resp+0x174/0x314 [mac80211]
    [<5974629e>] ieee80211_sta_rx_queued_mgmt+0x3f4/0x7f0 [mac80211]
    [<d91091c6>] ieee80211_iface_work+0x208/0x318 [mac80211]
    [<ac5fcae4>] process_one_work+0x22c/0x564
    [<f5e6d3b6>] worker_thread+0x44/0x5d8
    [<82c7b073>] kthread+0x150/0x154
    [<b43e1b7d>] ret_from_fork+0x14/0x2c
    [<794dff30>] 0x0

It is because 8192cu doesn't implement usb_cmd_send_packet(), and this
patch just frees the skb within the function to resolve memleak problem
by now. Since 8192cu doesn't turn on fwctrl_lps that needs to download
command packet for firmware via the function, applying this patch doesn't
affect driver behavior.
Reported-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/realtek/rtlwifi/rtl8192cu/hw.c

@@ -1540,6 +1540,8 @@ static bool usb_cmd_send_packet(struct ieee80211_hw *hw, struct sk_buff *skb)
    * This is maybe necessary:
    * rtlpriv->cfg->ops->fill_tx_cmddesc(hw, buffer, 1, 1, skb);
    */
+	dev_kfree_skb(skb);
+
 	return true;
 }