rt2800: add helpers for reading dma done index

For mmio we do not properlly trace dma done Q_INDEX_DMA_DONE index
for TX queues. That would require implementing INT_SOURCE_CSR_*_DMA_DONE
interrupts, what is rather not worth to do due to adding extra
CPU load (small but still somewhat not necessary otherwise).

We can just read TX DMA done indexes from registers directly. What
will be used by watchdog.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>