HID: hid-lg-g15: Add hw_brightness_changed support for the G510 keyboard

HID: hid-lg-g15: Add hw_brightness_changed support for the G510 keyboard

Add hw_brightness_changed support for the G510 keyboard, so that e.g.
GNOME will show an OSD notification when toggling the backlight on/off
with the button the keyboard.

Note that it is not possible to turn the backlight back on by writing
/sys/class/leds/.../brightness it can only be turned on by pressing
the button on the keyboard. To reflect this /sys/class/leds/.../brightness
will always report the last brightness value independent of the on/off
toggle built into the keyboard.

Signed-off-by: Hans de Goede <hansg@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.com>