IB/mlx4: Support PMA counters for IBoE

Use the per port counter attached to all QPs created on that port to
implement port level packets/bytes performance counters a la IB.
Derived from a patch by Eli Cohen <eli@mellanox.co.il>

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>