leds: leds-ns2: use devm_gpio_request_one

devm_gpio_request_one is device managed and makes error handling
and cleanup simpler.

Cc: Simon Guinot <sguinot@lacie.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>