If there is a "phy" handle the probe function returns with holding a
reference to that node. Make sure that in the fixed phy case there is
also held a reference to yield a consistant state.

Also add the corresponding of_node_put in the error path and the remove
function.

Fixes: 83895bed ("net: mvneta: add support for fixed links")
Fixes: c5aff182 ("net: mvneta: driver for Marvell Armada 370/XP network unit")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>