net: stmmac: selftests: Do not fail if PHY is not attached

If a PHY is not attached, we can still run the tests with MAC loopback
mode. Return -EOPNOTSUPP error code in PHY loopback test so that global
status is not a failure.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>