rdma: Add get per-port counter mode support

rdma: Add get per-port counter mode support

Add an interface to show which mode is active. Two modes are supported:
- "auto": In this mode all QPs belong to one category are bind automatically
  to a single counter set. Currently only "qp type" is supported;
- "manual": In this mode QPs are bound to a counter manually.

Examples:
$ rdma statistic qp mode
0/1: mlx5_0/1: qp auto off
1/1: mlx5_1/1: qp auto off
2/1: mlx5_2/1: qp auto type on
3/1: mlx5_3/1: qp auto off

$ rdma statistic qp mode link mlx5_0
0/1: mlx5_0/1: qp auto off

Signed-off-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>