net: mdio-octeon: Fix octeon_mdiobus_probe function for return values

This patch fixes a possible crash in the octeon_mdiobus_probe function
if the return values are not handled properly.

Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>