mwifiex: skip pending commands after function shutdown

During rmmod mwifiex_sdio processing FUNC_SHUTDOWN command is
sent to firmware. Firmware expcets only FUNC_INIT once WLAN
function is shut down.

Any command pending in the command queue should be ignored and
freed.

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