IB/mlx5: Extend debug control for CC parameters

This patch adds rtt_resp_dscp to the current debug controllability of
congestion control (CC) parameters.
rtt_resp_dscp can be read or written through debugfs.
If set, its value overwrites the DSCP of the generated RTT response.

Signed-off-by: Edward Srouji <edwards@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Link: https://lore.kernel.org/r/1dcc3440ee53c688f19f579a051ded81a2aaa70a.1676538714.git.leon@kernel.org
Signed-off-by: Leon Romanovsky <leon@kernel.org>