net: bcmgenet: fix reference counting for phy node

For the fixed phy setup make sure to not overwrite a valid value of
phy_dn (that is holding a reference to a phy-handle) and also acquire a
reference to the MAC node to consistently return with the phy_dn pointer
holding a reference.

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

Fixes: 9abf0c2b717a ("net: bcmgenet: use the new fixed PHY helpers")
Fixes: aa09677cba42 ("net: bcmgenet: add MDIO routines")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>