ASoC: davinci-mcasp: Add support for I2S format

The FS needs to be inverted in McASP compared to other supported formats.
Use a flag to indicate if the FS needs to be inverted.
At the same time fail when non supported format is asked since the default
case was anyways configuring McASP to a not valid format.

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