rt2x00: Fix RT3572 channel switch RFCSR 7 programming.

Align with the v2.5.0.0 Ralink RT3572 driver.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>