rt2x00: rt2800lib: hardcode TX mixer gain values for RT3593

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>