leds: leds-ss4200: Staticize nasgpio_led_get_attr()

nasgpio_led_get_attr() is used only in this file.
Fix the following sparse warning:

drivers/leds/leds-ss4200.c:200:5: warning: symbol 'nasgpio_led_get_attr' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>