ath9k: setup correct skb priority for nullfunc

After queue nullfunc for MCC case, we hit WARN_ON
in xmit.c:2398 while skb priority wasn't set.

Signed-off-by: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>