avr: fix divide when CLOCK_SECOND is not a power of two
avr: fix divide when CLOCK_SECOND is not a power of two

bluerover/6lbr,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,bluerover/6lbr,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki