rt2x00: Make rt2x00leds_register return void

rt2x00dev isn't interested in the rt2x00leds_register() value
anyway. So lets make it return void to even prevent people from
assuming there is anybody interested in the returnvalue.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>