samples: rtu_server: rework LED configuration
samples: rtu_server: rework LED configuration

Follow modern way to configure LEDs GPIO using
struct gpio_dt_spec and GPIO_DT_SPEC_GET.

Signed-off-by: Johann Fischer <64a5f06fca6b2d93d742efe564789153170f6198@nordicsemi.no>

Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr