netdev: mdio-octeon: Fix section mismatch errors.

We started getting:

WARNING: vmlinux.o(.data+0x20bd0): Section mismatch in reference from
the variable octeon_mdiobus_driver to the function
.init.text:octeon_mdiobus_probe()

This fixes it.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>