leds: gpio: Use GPIOF_OUT_INIT_LOW instead of hardcoded zero

Use the GPIO flag definition instead of a numeric literal, so the casual
reader grepping for GPIOF_ will find the GPIO flags used.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>