ASoC: davinci-mcasp: Add support for FIFO usage caused delay reporting

McASP have write and read FIFO, each 64 words deep.

From the WFIFOS/RFIFOS registers we can read the amount of data currently
in the FIFO which can be directly reported as delay.

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