mlxsw: spectrum_flower: use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
 drivers/net/ethernet/mellanox/mlxsw/spectrum_flower.c:543:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>