mac80211: fix PREQ processing and one small bug

1st) a PREQ should only be processed, if it has the same SN and better
metric (instead of better or equal).
2nd) next_hop[ETH_ALEN] now actually used to buffer
mpath->next_hop->sta.addr for use out of lock.

Signed-off-by: Marco Porsch <marco.porsch@siemens.com>
Acked-by: Javier Cardona <javier@cozybit.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>