{nl,cfg}80211: make peer link expiration time configurable

If a STA has a peer that it hasn't seen any tx activity
from for a certain length of time, the peer link is
expired. This means the inactive STA is removed from the
list of peers and that STA is not considered a peer again
unless it re-peers.  Previously, this inactivity time was
always 30 minutes.  Now, add it to the mesh configuration
and allow it to be configured.  Retain 30 minutes as a
default value.

Signed-off-by: Colleen Twitty <colleen@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>