rt2x00: Use saved BBP 25 and 26 values when configuring channel on RT3572.

This brings the rt2800 channel switching code for RT3572 closer to the
v2.5.0.0 Ralink RT3572 driver.

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