ath5k: Add support for antenna configuration

Support setting the antenna configuration via cfg/mac80211. At the moment only
allow the simple pre-defined configurations we already have (fixed antenna A/B
or diversity), but more advanced settings are possible to implement.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>