Move led_mode attribute from HID device to led-class devices and
rename it mode. This will also fix race condition by using
attribute-groups.

(cooloney@gmai.com: fix a typo in commit message)
Signed-off-by: NJanne Kanniainen <janne.kanniainen@gmail.com>
Reviewed-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NBryan Wu <cooloney@gmail.com>