ath10k: fix not registering airtime of 11a station with WMM disable

The tid of 11a station with WMM disable reported by FW is 0x10 in
tx completion. The tid 16 is mapped to a NULL txq since buffer
MMPDU capbility is not supported. Then 11a station's airtime will
not be registered due to NULL txq check. As a results, airtime of
11a station keeps unchanged in debugfs system.

Mask the tid along with IEEE80211_QOS_CTL_TID_MASK to make it in
the valid range.

Hardwares tested : QCA9984
Firmwares tested : 10.4-3.10-00047

Signed-off-by: Yibo Zhao <yiboz@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>