rt2x00: Fix transmit power troubles on some Ralink RT30xx cards

Some cards on Ralink RT30xx chipset not have correctly TX_MIXER_GAIN
value in them EEPROM/EFUSE. In this case, we must use default value,
but always used EEPROM/EFUSE value. As result we have tranmitt power
range from -10dBm to +6dBm instead 0dBm to +16dBm.

Correctly value in EEPROM/EFUSE is one or more for RT3070 and two or
more for other RT30xx chips.

Tested on Canyon CNP-WF518N1 usb Wi-Fi dongle and Jorjin WN8020 usb
embedded Wi-Fi module.
Signed-off-by: Alex A. Mihaylov <minimumlaw@rambler.ru>
Cc: stable@vger.kernel.org
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -5036,6 +5036,8 @@ static int rt2800_init_rfcsr(struct rt2x00_dev *rt2x00dev)
 
 	if (!rt2x00_rt(rt2x00dev, RT5390) &&
 	    !rt2x00_rt(rt2x00dev, RT5392)) {
+		u8 min_gain = rt2x00_rt(rt2x00dev, RT3070) ? 1 : 2;
+
 		rt2800_rfcsr_read(rt2x00dev, 17, &rfcsr);
 		rt2x00_set_field8(&rfcsr, RFCSR17_TX_LO1_EN, 0);
 		if (rt2x00_rt(rt2x00dev, RT3070) ||
@@ -5046,8 +5048,10 @@ static int rt2800_init_rfcsr(struct rt2x00_dev *rt2x00dev)
 				      &rt2x00dev->cap_flags))
 				rt2x00_set_field8(&rfcsr, RFCSR17_R, 1);
 		}
-		rt2x00_set_field8(&rfcsr, RFCSR17_TXMIXER_GAIN,
-				  drv_data->txmixer_gain_24g);
+		if (drv_data->txmixer_gain_24g >= min_gain) {
+			rt2x00_set_field8(&rfcsr, RFCSR17_TXMIXER_GAIN,
+					  drv_data->txmixer_gain_24g);
+		}
 		rt2800_rfcsr_write(rt2x00dev, 17, rfcsr);
 	}