net: fsl/fman: fix fixed-link-phydev reference leak

Make sure to drop the reference taken by of_phy_find_device() when
looking up a fixed-link phydev during probe.

Fixes: 57ba4c9b

 ("fsl/fman: Add FMan MAC support")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>