Fix incorrect computation of new channel number.
Fix incorrect computation of new channel number.

sudomesh/babeld,woniullb/babeld,jech/babeld,jech/babeld,tcatm/babeld,wlanslovenija/babeld,boutier/babeld,Gwendocg/babeldToS,dtaht/babeld-shortrtt-metrics,Drooids/babeld,Gwendocg/babeldToS,wlanslovenija/babeld