mt76: mt7921: introduce mt7921_dma_{enable,disable} utilities

Introduce mt7921_dma_enable and mt7921_dma_disable utilities routine in
order for code reusing between mt7921_dma_reset and mt7921_dma_init.
This is a preliminary patch to reset dma during device driver_own
request.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>