ASoC: davinci-mcasp: Fine tune and correct the DMA burst configuration

When the AFIFO is not enabled but more than one serializers are used the DMA
need to transfer number of words equal to active serializers when a DMA
request is generated.
When configuring the burst for the DMA avoid using value '1' for the burst
since it is going to enable additional logic in the DMA drivers. Burst '1'
means that the DMA should send/receive one word per DMA requests.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>