ASoC: imx-ssi: Remove mono support

Playing a mono track results in incorrect playback rate, ie, the audio
is played at a faster rate.

Remove mono support in the driver by setting 'channes_min' to dual-channel
and this allows mono tracks to be played correctly.

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