mwifiex: fix issues in driver unload path for USB chipsets

1) After driver load failure, clear 'card->adapter' instead of
card pointer so that card specific cleanup is performed later
when user unloads the driver.

2) Clear usb_card pointer in disconnect handler to avoid invalid
memory access when user unloads the driver after removing the
card.

Signed-off-by: Ujjal Roy <royujjal@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>