mac80211: stop plink timer only on mesh interfaces

Since mesh_plink_quiesce() would unconditionally delete
the plink timer, and the timer initialization was recently
moved into the mesh code path, suspending with a non-mesh
interface now causes a crash. Fix this by only deleting
the plink timer for mesh interfaces.

Reported-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Tested-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>