mac80211: fixes for mesh powersave logic

This patch fixes errors in the mesh powersave logic which
cause that remote peers do not get peer power mode change
notifications and mesh peer service periods (MPSPs) got
stuck.

When closing a peer link, set the (now invalid) peer-specific
power mode to 'unknown'.

Avoid overhead when local power mode is unchanged.

Reliably clear MPSP flags on peering status update.

Avoid MPSP flags getting stuck by not requesting a further
MPSP ownership if we already are an MPSP owner.

Signed-off-by: Marco Porsch <marco@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>