fsl/fman: check dereferencing null pointer

[ Upstream commit cc5d229a ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>