rt2x00: Add rt2870 device id

Add ID for Asus USB-N11 Wi-FI adapter. Tested by me.

Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>