rtw88: Use kfree_skb() instead of kfree()

sk_buff structs need to be freed with kfree_skb(), not kfree().

Fixes: b6c12908

 ("rtw88: Add wowlan net-detect support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>