ASoC: dapm: Remove incomplete stereo code

Stereo is not yet supported by dapm widgets, so remove stereo code from
snd_soc_dapm_get_volsw(), and warn if stereo controls are detected.

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