ethernet: fix min/max MTU typos

Fixes: d894be57

('ethernet: use net core MTU range checking in more drivers')
CC: Jarod Wilson <jarod@redhat.com>
CC: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>