mwifiex: add unregister_dev handler for usb interface

Clear the data pointer stored in USB interface structure in
this handler. This helps to return from mwifiex_usb_disconnect()
if driver deinitialization is already performed while handling
an error path for mwifiex_usb_probe().

USB8797 card gets enumerated twice. First enumeration is for
firmware download and second enumeration expects firmware
initialization. mwifiex_usb_probe() always takes care of
deinitialization for first enumeration after firmware download.

Also, this change matches our handling for SDIO and PCIe
interfaces.

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>