rt2x00: Add initial support for RT3370/RT3390 devices.

Modified from Eddy's patch by adding the RT3370 USB support as well.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Cc: Eddy Tsai <Eddy_Tsai@ralinktech.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>