leds: netxbig: convert to use the devm_ functions

This patch converts the leds-netxbig driver to use the devres functions
devm_gpio_request_one() and devm_led_classdev_register(). This allows to
simplify the code a bit.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>