ASoC: davinci-mcasp: Simplify and clean up the AFIFO configuration code

We can have more linear code flow by using variables in
mcasp_common_hw_param() related to the AFIFO configuration.

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