net/mlx4_en: Show PF own statistics via ethtool

Allow the user to observe the PF own statistics using ethtool with pf_
prefixed counter names.

Those counters are the PF statistics out of the overall port statistics.
Every PF QP is attached to a counter and the summary of those counters
is the PF statistics.

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>