net/mlx5_core: Add helper function to read virtual port counters

Added helper function to read 64bit virtual port Infiniband traffic
counters.

Signed-off-by: Meny Yossefi <menyy@mellanox.com>
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>