boards/nrf9160dk: fix LED macros
boards/nrf9160dk: fix LED macros

LED macros were inverted, so LEDn_ON would turn the LED off and LEDn_OFF
would turn it on.

jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,OlegHahm/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,OlegHahm/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,OlegHahm/RIOT,miri64/RIOT,ant9000/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT