ASoC: ti: davinci-mcasp: Remove legacy dma_request parsing

The legacy dma_request (which was holding the DMA request number) is no
longer in use for a long time.
All legacy platforms has been converted to dma_slave_map.

Remove it along with the DT parsing to get tx_dma_channel and
rx_dma_channel.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20201106072551.689-3-peter.ujfalusi@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>