staging: rtl8188eu: Remove struct rtw_usb_drv

struct rtw_usb_drv contains only one member usb_driver structure,
so no use of keeping it.

Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>