ASoC: mc13783: Remove mono support

Playing a mono track on a mc13783 codec results in incorrect playback rate.

Remove mono support so that a mono track can be played correctly.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Gaëtan Carlier <gcembed@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>