IB/mlx5: Support congestion related counters

This patch adds support to query the congestion related hardware counters
through new command and links them with other hw counters being available
in hw_counters sysfs location.

In order to reuse existing infrastructure it renames related q_counter
data structures to more generic counters to reflect q_counters and
congestion counters and maybe some other counters in the future.

New hardware counters:
 * rp_cnp_handled - CNP packets handled by the reaction point
 * rp_cnp_ignored - CNP packets ignored by the reaction point
 * np_cnp_sent    - CNP packets sent by notification point to respond to
                     CE marked RoCE packets
 * np_ecn_marked_roce_packets - CE marked RoCE packets received by
                                notification point

It also avoids returning ENOSYS which is specific for invalid
system call and produces the following checkpatch.pl warning.

WARNING: ENOSYS means 'invalid syscall nr' and nothing else
+		return -ENOSYS;

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Eli Cohen <eli@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>