ASoC: davinci-mcasp: Move the FS polarity change out from the switch case

FS polarity can be either rising or falling edge in McASP. Instead of
accessing the registers in every switch/case set a flag and write the
registers after the switch for the invert configuration.
This change will help when adding support for more formats also.

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