octeon/cnmac: assume 1Gbps in the default case

octeon/cnmac: assume 1Gbps in the default case

avoids a division by 0 when up'ing the non-working cnmac2 on er-poe

feedback from visa@:
"The cnmac2 link is connected to a discrete Ethernet switch chip on the
PoE, and this link does not carry RGMII in-band PHY status.  If the code
were to support such hardware designs in the future, something like this
patch would be needed."

this diff from miod@, same diff from visa@ in 2017
(cf https://marc.info/?l=openbsd-bugs&m=151017517115440&w=2)
ok visa@