rt2x00: Fix RFCSR 12 & 13 programming on RT3572 channel switching.

Align with v2.5.0.0 Ralink RT3572 driver for 2.4GHz band channel switch.

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>