mt76: mt7915: add a fixed AC queue mapping

In MT7915, hardware queue map is flexible. However, certain firmware modules
like MU and U-APSD presume a fixed queue order to adapt some devices that have
DMA scheduler with a strict order, so this patch can help in the long run.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>