This solution is only for network cards managed by NetworkManager (under SLED12 SP4).

Code:
#!/bin/bash
#########################################################################
# /etc/NetworkManager/dispatcher.d/ethtool.sh
#########################################################################

INTERFACE=$1
ACTION=$2

#
# paranoia
#
umask 022
PATH=/bin:/usr/bin
export PATH

case "$2" in
        up)
                /usr/sbin/ethtool --set-ring <devname> 
                ;;

        *)
                exit 0
esac
Don't forget the execute flag:

# chmod u+x,g+x,o+x /etc/NetworkManager/dispatcher.d/ethtool.sh