dpaa2-eth: Use PTR_ERR_OR_ZERO where appropriate

Use PTR_ERR_OR_ZERO instead of PTR_ERR in cases where
zero is a valid input. Reported by smatch.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>