mac80211: use helper function to access ieee802_1d_to_ac[]

cleanup patch to make use of ieee80211_ac_from_tid() to retrieve ac from
ieee802_1d_to_ac[]

Signed-off-by: Amadeusz Sławiński <amadeusz.slawinski@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>