ASoC: sgtl5000: Do not disable regulators in SND_SOC_BIAS_OFF

Disabling the SGTL5000 through regulators would certainly save more
power than simply disabling the reference voltages as described in the
data sheet, but won't properly restore things on resume.

This driver does not support active regulators. So we simply disable the
reference bias currents.

Signed-off-by: Eric Nelson <eric@nelint.com>
Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>