mac80211: factor out 802.11 header building code

Factor out the 802.11 header building code from the xmit function
to be able to use it separately in a later commit.

While at it, fix up some documentation.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>