mac80211: add helper to free TX skb

Drivers that need to drop a frame before it
can be transmitted will usually simply free
that frame. This is currently fine, but in
the future it'll be needed to tell mac80211
about this case, so add a new routine that
frees a TX skb.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>