mac80211: add explicit AP/GO driver operations

mac80211: add explicit AP/GO driver operations

Depending on the driver, a lot of setup may be
necessary to start operating as an AP, some of
which may fail. Add an explicit AP start driver
method to make such failures easier to handle,
and add an AP stop driver method for symmetry.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>