ASoC: OMAP: Fix DSP_B format in OMAP McBSP DAI driver

The DSP format wasn't still correct in OMAP McBSP DAI even after the commit
bd25867a.

Thanks to Peter Ujfalusi <peter.ujfalusi@nokia.com> for noticing and being
part of the fix. Now the FS length definition is more clear by defining
it with  FWID(0).

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>