nl80211: export supported commands

This makes nl80211 export the supported commands (command groups)
per wiphy so userspace has an idea what it can do -- this will be
required reading for userspace when we introduce auth/assoc /or/
connect for older hardware that cannot separate auth and assoc.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>