leds: leds-lt3593: use devm_gpio_request_one

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

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Bryan Wu <cooloney@gmail.com>