ASoC: davinci-mcasp: Simplify FIFO configuration code

The FIFO registers base address is different in dm646x compared to newer
SoCs with McASP IP. Instead of using two paths (switch/case) to handle the
difference we can simply pick the correct base address beforehand and use
offsets to address the register we need to configure.
With this change the indentation depth can be reduced as well.

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