mac80211: fix description comment of ieee80211_subif_start_xmit

The function description claimed that on error the skb isn't
freed even though it is, and stated return values that are
different than what really happens in the code.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>