mac80211: stringify mesh peering events

Convert mesh peering events into strings and make the
debug output a little easier to read. Also stop printing
the llid and plid since these don't change across peering
states and are random numbers anyway so they just amount
to noise.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>