rtl8xxxu: Simplify locking of a skb list accesses

The 'c2hcmd_lock' spinlock is only used to protect some __skb_queue_tail()
and __skb_dequeue() calls.
Use the lock provided in the skb itself and call skb_queue_tail() and
skb_dequeue(). These functions already include the correct locking.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/8bcec6429615aeb498482dc7e1955ce09b456585.1617613700.git.christophe.jaillet@wanadoo.fr