ASoC: max9867: Fix power management

[ Upstream commit 29f58ff0

 ]

Implement set_bias_level to drive shutdown bit, so device is
put to sleep when unused.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>