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: NGertjan van Wingerde <gwingerde@gmail.com>
Acked-by: NStanislaw Gruszka <sgruszka@redhat.com>
Acked-by: NIvo van Doorn <IvDoorn@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>