Otherwise, if we have two PTP ports for the same interface, like
Ethernet and UDP, we are not able to tell which message is for which.
This now works as expected:
./ppsi -C "port eth0; iface eth0; role master; proto raw; diagnostics 111" \
-C "port eth0u; iface eth0; role master; proto udp; diagnostics 111"
Signed-off-by: Alessandro Rubini <email@example.com>