ASoC: sgtl5000: Convert mic bias to a supply widget

No current users and it's the last user of MICBIAS_E().

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Acked-by: Zeng Zhaoming <zengzm.kernel@gmail.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>