rt2x00: Initial support for RT5370 USB devices.

Add necessary RF chipset define and basic support for these devices.

Tested-by: Juan Carlos Garza <juancarlosgarza@gmail.com>
Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>