rt2x00: Add device ID for RT539F device.

Reported-by: Wim Vander Schelden <wim@fixnum.org>
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>