mt76: mt7921: move mt7921_dma_reset in dma.c

Move mt7921_dma_reset routine in dma.c and make mt7921_dma_prefetch
static. Moreover add force parameter to mt7921_dma_reset signature.
This is a preliminary patch to reset dma mt7921_mcu_drv_pmctrl.

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>