ASoC: rt286: Fix capture volume setting issue

The purpose of rt286_adc_event is to mute/numnte the ADC mixer.
However, it will also set the capture volume to default value.
As a result, "ADC0 Capture Volume" is not working if it is set
before capture start. This patch remove rt286_adc_event and add
"ADC0 Capture Switch" to mute/unmute ADC mixer.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>