mac80211: clarify alignment comment

The comment says something about __skb_push(), but that
isn't even called in the code any more. Looking at the
git history, that comment never even made sense when it
was still called, so just replace that part to note it
still works even when align isn't 0 or 2.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>