mac80211: fix the increment of unicast/multicast counters for forwarded PREQ

Forwarded PREQ is either unicast or multicast. The appropriate counters
should be incremented accordingly.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>