fsl/fman: fix dereference null return value

Check before using returned value to avoid dereferencing null pointer.

Fixes: 18a6c85f ("fsl/fman: Add FMan Port Support")
Signed-off-by: Florinel Iordache <florinel.iordache@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>