mac80211: fix incorrect mesh header length

This should have been updated at the same time we were transitioning from 3 byte
to 4 byte mesh sequence number. Pointed out by Johannes Berg.

Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

diff --cc net/mac80211/mesh.c
Simple merge

diff --cc net/mac80211/util.c
Simple merge