net/mlx4_core: Add helper to query counters

This is an infrastructure step for querying VF and PF counters.

This code was in the IB driver, move it to the mlx4 core driver
so it will be accessible for more use cases.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>