ASoC: davinci-mcasp: Fix dra7 DMA offset when using CFG port

The TX and RX offset is different for each serializers when using the CFG
port for DMA access.
When using the CFG port only one serializer can be used per direction so
print error message and only configure the first serializer's offset.

Reported-by: Misael Lopez Cruz <misael.lopez@ti.com>
Suggested-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>