HID: lg-g15: Add support for controlling the G510's RGB backlight

Note that the keyboard has a backlight on/off toggle button. If the
backlight is turned off through that button, then any changes we make
will be ignored and we cannot turn it back on again from the host.

To workaround this we write the last set RGB values when we receive an
event indicating that the backlight has been turned on again.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>