mac80211: add option for setting skb flags before xmit

Allows setting of an skb's flags - if needed - when calling
ieee80211_subif_start_xmit().

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