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>