mac80211: allow drivers to request SM PS mode change

Sometimes drivers have more information than the
stack about how their antennas/chains are used,
and may require that the SM PS mode be changed.
This could happen, for example, when detecting
that the user disconnected an antenna. Thus this
patch introduces API to allow drivers to request
SM PS mode changes.

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