ASoC: mxs-pcm: Set SNDRV_PCM_INFO_HALF_DUPLEX

The MXS SAIF is only half-duplex so set the SNDRV_PCM_INFO_HALF_DUPLEX flag for
the PCM in order to prevent playback and capture from running at the same time.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>