mac80211: Add antenna configuration

Allow antenna configuration by calling driver's function for it.

We disallow antenna configuration if the wiphy is already running, mainly to
make life easier for 802.11n drivers which need to recalculate HT capabilites.

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