rt2800: validate step value for temperature compensation

commit bf7e1abe434ba9e22e8dc04a4cba4ab504b788b8 upstream.

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>
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/net/wireless/rt2x00/rt2800lib.c b/drivers/net/wireless/rt2x00/rt2800lib.c
index 6c0a12e..2ce6bf5b 100644
--- a/drivers/net/wireless/rt2x00/rt2800lib.c
+++ b/drivers/net/wireless/rt2x00/rt2800lib.c
@@ -2235,7 +2235,7 @@
 	/*
 	 * Check if temperature compensation is supported.
 	 */
-	if (tssi_bounds[4] == 0xff)
+	if (tssi_bounds[4] == 0xff || step == 0xff)
 		return 0;
 
 	/*