leds: leds-gpio-register: Supply description for param 'id'

Fixes the following W=1 kernel build warning(s):

 drivers/leds/leds-gpio-register.c:24: warning: Function parameter or member 'id' not described in 'gpio_led_register_device'

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Cc: linux-leds@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>