mlxsw: spectrum_mr: Use flexible-array member instead of zero-length array

Suppresses the following coccinelle warning:

drivers/net/ethernet/mellanox/mlxsw/spectrum_mr.c:18:15-19: WARNING use flexible-array member instead

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>