mac80211: fix mesh fail_avg check

Mesh failure average never be more than 100. Only in case of
fixed path, average will be more than threshold limit (95%).
With recent EWMA changes it may go upto 99 as it is scaled to
100. It make sense to return maximum metric when average is
greater than threshold limit.

Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>