ASoC: omap-mcbsp: Use sDMA packet mode instead of frame mode

When McBSP is configured in threshold mode we can use sDMA packet mode in
all cases.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Tested-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>