rt2x00: Correctly set txmixer_gain in RT3572 channel switching.

Align with the v2.5.0.0 Ralink RT3572 driver.

Save the EEPROM txmixer_gain values inside the rt2800 driver data structure
and use it throughout the code.

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>