mt76: mt7915: add .offset_tsf callback

It's much more accurate than .get_tsf + .set_tsf, and switch to use
mt76_rmw to operate tsf registers.

Tested-by: Xing Song <xing.song@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>