mwifiex: fix NULL packet downloading issues

This patch makes sure that skb is freed after downloading
NULL packet in error cases. Also, USB chipsets return
-EINPROGRESS after downloading packets, they are freed in
USB completion handler later. We will add missing change
to set tx_lock_flag for USB which blocks further packets.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>