mac80211: clean up IEEE80211_FC use

Really doesn't need to be defined four times.

Also, while at it, remove a useless macro (IEEE80211_ALIGN32_PAD)
and a function prototype for a function we don't actually have
(ieee80211_set_compression.)

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