ath6kl: Fix WMI message structure for AP_SET_PVB

There is a 2-octet reserved field between the flag and aid fields. Fix
that to make the target actually behave as requested.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>