mwifiex: use USB core's soft_unbind option

This option allows driver to finish pending operations in
disconnect handler by not killing URBs after usb_deregister
call.

We will get rid of global pointer 'usb_card' by moving code
from cleanup_module() to disconnect(). This will help to match
with 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>