mlxsw: spectrum_dpipe: Fix sparse warnings

drivers/net/ethernet/mellanox/mlxsw//spectrum_dpipe.c:221:52: warning:
Using plain integer as NULL pointer
drivers/net/ethernet/mellanox/mlxsw//spectrum_dpipe.c:221:74: warning:
Using plain integer as NULL pointer

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>