cfg80211: restructure AP/GO mode API

The AP/GO mode API isn't very clearly defined, it
has "set beacon" and "new beacon" etc.

Modify the API to the following:
 * start AP -- all settings
 * change beacon -- new beacon data
 * stop AP -- stop AP mode operation

This also reflects in the nl80211 API, rename
the commands there correspondingly (but keep
the old names for compatibility.)

Overall, this makes it much clearer what's going
on in the API.

Kalle developed the ath6kl changes, I created
the rest of the patch.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>