[iwm] Allow listening on both chains/atennas to get diversity.

This might improve throughput slightly when far from the accesspoint,
apparently by allowing the firmware to listen on either of the two
antennas (if there are two, i.e. on 7260/7265/8260), whichever has
a better reception.

Obtained from: dragonflybsd.git 3b7fc5aac51f81062da0a2c8fdac23e683fbd548