iwlwifi: find the correct first antenna

We can not assume antenna "A" is the first valid anttena for
all the NIC. Need to make sure choice the correct antenna based on
h/w configuration for transmit to avoid sending frame on invalid
antenna

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>