mwifiex: cancel cmd timer and free curr_cmd in shutdown process

curr_cmd points to the command that is in processing or waiting
for its command response from firmware. If the function shutdown
happens to occur at this time we should cancel the cmd timer and
put the command back to free queue.

Cc: <stable@vger.kernel.org> # 3.8
Tested-by: Marco Cesarano <marco@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>