iwlwifi: use mac80211 AC defines

Instead of hardcoding the numbers that must
match mac80211, use the constants. Not that
this means we could change the constants,
but at least this way it's clearer.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>