ALSA: hda - Fix mute-LED VREF value for new HP laptops

The new HP laptops turns off the mute LED with VREF50 or VREF80, but
not in HIZ unlike the previous models.  Since VREF50 (also 80) works
with the previous models, let's use VREF50 for all.

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