rt2800: fix wrong TX power compensation

We should not do temperature compensation on devices without
EXTERNAL_TX_ALC bit set (called DynamicTxAgcControl on vendor driver).
Such devices can have totally bogus TSSI parameters on the EEPROM,
but still threaded by us as valid and result doing wrong TX power
calculations.

This fix inability to connect to AP on slightly longer distance on
some Ralink chips/devices.
Reported-and-tested-by: Fabien ADAM <id2ndr@crocobox.org>
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -3407,6 +3407,13 @@ static int rt2800_get_gain_calibration_delta(struct rt2x00_dev *rt2x00dev)
 	u8 step;
 	int i;
 
+	/*
+	 * First check if temperature compensation is supported.
+	 */
+	rt2800_eeprom_read(rt2x00dev, EEPROM_NIC_CONF1, &eeprom);
+	if (!rt2x00_get_field16(eeprom, EEPROM_NIC_CONF1_EXTERNAL_TX_ALC))
+		return 0;
+
 	/*
 	 * Read TSSI boundaries for temperature compensation from
 	 * the EEPROM.