ALSA: hda: add mute LED support for HP EliteBook 840 G4

Tested with 4.19.9.

v2: Changed from CXT_FIXUP_MUTE_LED_GPIO to CXT_FIXUP_HP_DOCK because
    that's what the existing fixups for EliteBooks use.

Signed-off-by: Mantas Mikulėnas <grawity@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>