mac80211: allow drivers to request SMPS off

Previously we were warning and using automatic when a driver sent an
update request with SMPS off.  This patch makes it possible for
drivers to disable SMPS at runtime, for whatever reason.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>