wifi: mt76: mt7915: rework mt7915_dma_reset()

Reuse mt7915_dma_disable() to reduce duplicated code.
This is a preliminary patch to enable full system reset.

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