net: mscc: ocelot: use a consistent cookie for MRP traps

The driver uses an identifier equal to (ocelot->num_phys_ports + port)
for MRP traps installed when the system is in the role of an MRC, and an
identifier equal to (port) otherwise.

Use the same identifier in both cases as a consolidation for the various
cookie values spread throughout the driver.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>