iwlwifi: fix TX power antenna access

Since my commit
  iwlwifi: use valid TX/RX antenna from hw_params
the config values are pure overrides, not the
real values for all hardware. Therefore, the
EEPROM TX power reading code checks the wrong
values, it should check the hw_params values.

Cc: stable@kernel.org [3.4]
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>