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: NGabor Juhos <juhosg@openwrt.org>
Acked-by: NStanislaw Gruszka <stf_xl@wp.pl>
Acked-by: NGertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -6365,6 +6365,9 @@ static u8 rt2800_get_txmixer_gain_24g(struct rt2x00_dev *rt2x00dev)
 {
 	u16 word;
 
+	if (rt2x00_rt(rt2x00dev, RT3593))
+		return 0;
+
 	rt2800_eeprom_read(rt2x00dev, EEPROM_TXMIXER_GAIN_BG, &word);
 	if ((word & 0x00ff) != 0x00ff)
 		return rt2x00_get_field16(word, EEPROM_TXMIXER_GAIN_BG_VAL);
@@ -6376,6 +6379,9 @@ static u8 rt2800_get_txmixer_gain_5g(struct rt2x00_dev *rt2x00dev)
 {
 	u16 word;
 
+	if (rt2x00_rt(rt2x00dev, RT3593))
+		return 0;
+
 	rt2800_eeprom_read(rt2x00dev, EEPROM_TXMIXER_GAIN_A, &word);
 	if ((word & 0x00ff) != 0x00ff)
 		return rt2x00_get_field16(word, EEPROM_TXMIXER_GAIN_A_VAL);