RDMA/mlx5: Use proper spec flow label type

Flow label is defined as u32 in the in ipv6 flow spec, but
used internally in the flow specs parsing as u8. That was
causing loss of part of flow_label value.

Fixes: 2d1e697e ('IB/mlx5: Add support to match inner packet fields')
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Daria Velikovsky <daria@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>