ALSA: hda - Set TLV_DB_SCALE_MUTE bit for cx5051 vmaster

Conexnat HD-audio driver has a workaround for cx5051 (aka CX20561)
chip to add fake mute controls to each amp (commit 3868137e).  This
implies the minimum-as-mute TLV bit in TLV for each corresponding
control.  Meanwhile we build the virtual master from these, but the
TLV bit is missing, even though the slaves have it.

This patch simply adds the missing TLV_DB_SCALE_MUTE bit for vmaster,
as already done in patch_sigmatel.c.

Signed-off-by: Takashi Iwai <tiwai@suse.de>