[ALSA] Fix volume control for the AK4358 DAC

Fix volume control for the AK4358 DAC.
The attenuation control registers of the AK4358 use only 7bit for the
volume, the msb is used to enable attenuation output.  Without this
patch there are 256 volume levels the lower 128 of which are mute.

Signed-off-by: Jochen Voss <voss@seehuhn.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>