ASoC: rt5640: add Mono ADC Capture Switch control

Mono ADC Capture Switch control is missing in the driver. So, add it.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Tested-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>