cpu/lpc23xx: RTC: cleanup
cpu/lpc23xx: RTC: cleanup

 - reduce indentation
 - sanitize logic in rtc_set_alarm()
 - ILR register is clear-on-write, writing 0 has no effect

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