ALSA: hda/realtek - Fixup mute led on HP Spectre x360

This patch adds the mute LED control for HP Spectre x360 Kabylake
model.  The mute LED is controlled via VREF bits on NID 0x1b, so we
need a new fixup function.

Note that this doesn't fix the other issues like the missing speaker
output on the machine.  They will be addressed by later patches.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=189331


Signed-off-by: Tom Briden <tom@decompile.me.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>