mac80211: set forwarding in mesh capability info

Set the Forwarding bit in Mesh Capability Info according
to dot11MeshForwarding as defined in IEEE 802.11-2012
section 8.4.2.100.8.

Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>