ASoC: cs4270: Remove mono support

According to cs4270 datasheet, there is no reference to mono mode.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>