ASoC: tlv320aic3x: Remove mono support

Playing a mono stream on the TLV320AIC3x results in too fast playback rate.

Remove mono support so that mono streams can be played correctly on this codec.

Tested with imx-ssi (i.MX25) and TLV320AIC3104.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>