mac80211: Update the station failed frames average when minstrel is used.

The fail_avg value is used to compute the mesh metric, and was only being set
by the pid rate control module. This fixes the mesh path selection mechanism
for cards that use mistrel for rate control.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>