net: fsl/fman: fix phydev reference leak

Make sure to drop the reference taken by of_phy_find_device() during
initialisation when later freeing the struct fman_mac.

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>