myri10ge: fix potential infinite loop in enable_ecrc

Fix another potential for an infinite loop while looking for the
root port in myri10ge_enable_ecrc().

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Andrew Gallatin <gallatin@myri.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>