net/mlx5: Fix DCT creation bad flow

In case the DCT creation command has succeeded a DRAIN must be issued
before calling DESTROY.

In addition, the original code used the wrong parameter for the DESTROY
command, 'in' instead of 'din', which caused another creation try instead
of destroying.

Cc: <stable@vger.kernel.org> # 4.15
Fixes: 57cda166 ("net/mlx5: Add DCT command interface")
Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Reviewed-by: Artemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>