ASoC: davinci-mcasp: Fix DIT only McASP instance support

ASoC: davinci-mcasp: Fix DIT only McASP instance support

One of the McASP instances in DM646x line of DMSoC only supports DIT mode.
This means that the given IP does not have support for rx and all the rx
related resources are missing, like irq and DMA request.
The driver should not fail if any or all  of the RX resource is missing
when the op_mode is set to DIT mode.
Since RX is not possible in DIT mode, we can just ignore the rx resources
when the McASP is used in DIT mode.

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