platform/x86: system76_acpi: Add attribute group for kb_led_color

Create the attribute groups for kb_led_color and set the `groups` field
in kb_led. While touching it, also change its show method to use
sysfs_emit() instead of sprintf().

Signed-off-by: Tim Crawford <tcrawford@system76.com>
Link: https://lore.kernel.org/r/20211006202202.7479-5-tcrawford@system76.com


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>