Revert 0865a75d(ASoC: imx-ssi: Remove mono support).

The bug this patch is meant to solve doesn't occur in Visstrim_M10 boards.
Furthermore, after applying this patch sound in Visstrim_M10 is played
at slower rates.
Signed-off-by: NJavier Martin <javier.martin@vista-silicon.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>