mwifiex: fix memory leak in mwifiex_send_processed_packet()

Memory is leaked after downloading already processed packet. This
patch fixes the problem by freeing returned skb. Other transmit
paths don't have this problem.

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