RDMA/mlx5: Change scatter CQE flag to be set like other vendor flags

In similar way to wqe_sig, the scat_cqe was treated differently from
other create QP vendor flags. Change it to be similar to other flags
and use flags_en mechanism.

Link: https://lore.kernel.org/r/20200427154636.381474-17-leon@kernel.org


Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>