wireless: rt2x00: rt2800usb: identify Allwin devices

Applied common sense, no info from the manufacturer:

(0x8516, 0x2070) is RT2070
(0x8516, 0x2770) is RT2770
(0x8516, 0x2870) is RT2870
[...]

Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>