bytcr-rt5640: Add support for controlling a speaker-mute LED

bytcr-rt5640: Add support for controlling a speaker-mute LED

Add support for controlling a speaker-mute LED using the new snd_ctl_led
kernel module for generic mute LED control.

Tested on a HP Pavilion X2 10-n000nd and a HP Pavilion X2 10-p002nd both of
which have a detachable USB keyboard with a speaker mute LED embedded in
the audio-mute button.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>