leds: leds-gpio: use devm_gpio_request_one

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

Cc: Raphael Assenat <raph@8d.com>
Cc: Trent Piepho <tpiepho@freescale.com>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Bryan Wu <cooloney@gmail.com>