mt76: mt7915: set VTA bit in tx descriptor

The VTA (Valid for Txd Arrival time) bit should be set in tx descriptor,
which is necessary for WM fw to schedule SPL (station priority list)
normally before generating txcmd.
If it's not set, some unexpected wcids may be involved in SPL.

Suggested-by: Nelson Chang <nelson.chang@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>