The reference code uses hardcoded zero TX mixer gain value
for RT3593. Do the same in the rt2x00 driver.

Based on the Ralink DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver.

Reference:
  NICReadEEPROMParameters in common/rtmp_init.c
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>