leds: gpio: fix an unhandled error case in create_gpio_led

gpiod_get_value_cansleep returns 0, 1, or an error code.
So far errors are not handled and treated the same as 1.
Change this to bail out if an error code is returned and
remove the double negation.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>