mt76: move mt76_qsel definition in dma.h

Move mt76_qsel definition in dma.h in order to be reused in mt76x0
driver. Moreover remove empty mt76x2_dma.h header file

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>