fsl/fman: check dereferencing null pointer

Add a safe check to avoid dereferencing null pointer

Fixes: 57ba4c9b ("fsl/fman: Add FMan MAC support")
Signed-off-by: Florinel Iordache <florinel.iordache@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>