net/mlx5e: Expose flow control counters to ethtool

Just like per prio counters, the global flow counters are queried from
per priority counters register.
Global flow control counters are stored in priority 0 PFC counters.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>