net: mvpp2: cleanup probed ports in the probe error path

This patches fixes the probe error path by cleaning up probed ports, to
avoid leaving registered net devices when the driver failed to probe.

Fixes: 3f518509

 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>