HID: hid-lg-g15: Use standard multicolor LED API

Replace the custom "color" sysfs attribute with the standard multicolor
LED API.

This also removes the code for the custom "color" sysfs attribute,
the "color" sysfs attribute was never documented so hopefully, it is not
used by anyone.

If we get complaints, we can re-add the "color" sysfs attribute as
a compatibility wrapper setting the subleds intensity.

Signed-off-by: Kate Hsuan <hpa@redhat.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>