mac80211: allow to set smps mode to OFF in AP mode

In managed mode, we should not ask for OFF mode because the
power settings may still require DYNAMIC. In AP mode, this
should be allowed since the default settings is OFF and
AUTOMATIC is not allowed.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>