mac80211: don't reinit rate control when mesh sta exists

This fixes some unintended resets of the rate control statistics
when minstrel_ht is used resulting in non-optimal throughput on mesh
links.

Tested-by: Emanuel Taube <emanuel.taube@gmail.com>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>