RDMA/mlx5: Remove duplicated assignment to resp.response_length

RDMA/mlx5: Remove duplicated assignment to resp.response_length

The assignment to resp.response_length is never read since it is being
updated again on the next statement. The assignment is redundant so
removed it.

Fixes: a645a89d9a78 ("RDMA/mlx5: Return ECE DC support")
Link: https://lore.kernel.org/r/20200604143902.56021-1-colin.king@canonical.com
Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>