mac80211: defines for AC numbers

In many places we've just hardcoded the
AC numbers -- which is a relic from the
original mac80211 (d80211). Add constants
for them so we know what we're talking
about.

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