rt2800: validate step value for temperature compensation

Some hardware has correct (!= 0xff) value of tssi_bounds[4] in the
EEPROM, but step is equal to 0xff. This results on ridiculous delta
calculations and completely broke TX power settings.
Reported-and-tested-by: Pavel Lucik <pavel.lucik@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/rt2x00/rt2800lib.c

@@ -2449,7 +2449,7 @@ static int rt2800_get_gain_calibration_delta(struct rt2x00_dev *rt2x00dev)
 	/*
 	 * Check if temperature compensation is supported.
 	 */
-	if (tssi_bounds[4] == 0xff)
+	if (tssi_bounds[4] == 0xff || step == 0xff)
 		return 0;
 
 	/*