net: bcmgenet: fix phydev reference leak

Make sure to drop the reference taken by of_phy_find_device() when
initialising MOCA PHYs.

Fixes: 6ac9de5f ("net: bcmgenet: Register link_update callback for
all MoCA PHYs")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>