iwlwifi: use valid TX/RX antenna from hw_params

It's not really a good idea to write to the
global static configuration. Use the valid
TX/RX antenna information only from the HW
params struct except in the case where the
values from the config are used to override
the values from the EEPROM.

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>