iwlagn: remove Kelvin support

Only 5150 series devices report their temperature
in Kelvin, and for those we already convert it to
Celsius when storing into priv->temperature, so
there's no way priv->temperature will ever be in
Kelvin. Remove support for this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>