mac80211: rework TX filtered frame code

This reworks the code for TX filtered frames, splitting it out to
a new function to handle those cases, making the clear instruction
a flag and renaming a few things to be easier to understand and
less Atheros hardware specific. Finally, it also makes the comments
explain more.

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