mwifiex: fix driver unload problem for usb chipsets

We have usb_deregister() call in our rmmod routine. deauth,
shutdown etc. commands will be sent to FW later when bus
driver calls disconnect handler.

This mechanism works fine with SDIO and PCIe interfaces, but
there is an issue with USB.

USB bus driver returns all URBs submitted for receiving data
and command response immediately after usb_deregister() with
failure status. Hence we don't send deauth, shutdown etc.
command to firmware.

The problem is fixed by moving code from disconnect handler to
rmmod routine for USB interface.

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>