mt76: mt7615: implement DMA support for MT7622

MT7622 does not have the CR4 microcontroller sitting in the data path.
Because of that, it uses the chip's native tx descriptor format instead of
something parsed and converted by the firmware.

Co-developed-by: Shayne Chen <shayne.chen@mediatek.com>
Co-developed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>