mt76: mt7915: report tx rate directly from tx status

Report tx rate from tx status packets instead of receving periodic mcu
event. This improves flexibility, accuracy and AQL performance, and
simplifies code flow for better readability.

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