[ALSA] soc - Mono voice playback volume for WM8753

Voice playback volume is in register bits 0:2, not 4:6.

From: Mike Montour <mail@mmontour.net>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Werner Almesberger <werner@openmoko.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>