rtw88: pci: use ieee80211_ac_numbers instead of 0-3

AC numbers are defined as enum in mac80211, use them instead of bare
0-3 indexing.

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>