ASoC: sgtl5000: fix buggy 'Capture Attenuate Switch' control

The SGTL5000 Capture Attenuate Switch (or "ADC Volume Range Reduction"
as it is called in the manual) is single bit only.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>