mwifiex: reset card->adapter during device unregister

card->adapter gets initialized in mwifiex_register_dev(). As it's not
cleared in mwifiex_unregister_dev(), we may end up accessing the memory
which is already free in below scenario.

Scenario: Driver initialization is failed due to incorrect firmware or
some other reason. Meanwhile device reboot/unload occurs.

This is safe, now that we've properly synchronized suspend() and
remove() with the FW initialization thread; now that code can simply
check for 'card->adapter == NULL' and exit safely.

Signed-off-by: Xinming Hu <huxm@marvell.com>
Tested-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>