mac80211: don't re-init rate control when receiving mesh beacon

Rate control is re-initialized whenever a beacon from a mesh
peer received, breaking the algorithms and resulting in low
performance. Return early from mesh_peer_init if we already
established a link with this peer to avoid this.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
[clarify commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>