ASoC: Fix lm4857 control

Commit 4eaa9819

 changed semantics of
private_value member of kcontrol. This resulted in inability to control
amplifier and subsequently in very low output volume.

Tested-by: Johannes Schauer <josch@pyneo.org>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>