RDMA/mlx5: Set QP subtype immediately when it is known

There is no need to delay QP subtype assignment to the end of the
create_qp() function and it is better to move it to be immediately
after it is checked so we would be able to rewrite later checks
to be based on it and not on over-written struct ib_qp_init_attr.

Link: https://lore.kernel.org/r/20200427154636.381474-7-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>