mwifiex: Remove redundant skb_queue_empty checks

The check of skb list empty before calling skb_peek and skb_dequeue is
redundant. These functions returns NULL if the list is empty.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>