mwifiex: add PCIe shutdown handler to avoid system hang on reboot

If reboot command is issued when device is in connected state,
system hangs while booting. This issue is fixed by doing cleanup
in shutdown handler.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>