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: NFabio Estevam <fabio.estevam@freescale.com>
Tested-by: NGaëtan Carlier <gcembed@gmail.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>