mwifiex: stop AP at shutdown time

Deauth is sent to AP when the device is acting as station at
shutdown time. Similarly we should stop AP operation also.
mwifiex_deauthenticate() takes care closing the connection
based on provided interface type.

Add a new function to simplify the code.

Reported-by: Avery Pennarun <apenwarr@gmail.com>
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>