HID: elan: Set default_trigger for the mute LED

The mute LED should be automatically turned on/off based
on the audio-card's mixer settings.

Add the standardized default-trigger name for this, so that the alsa
code can turn the LED on/off as appropriate (on supported audio cards).

Reviewed-by: Marek Behún <kabel@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>