net/mlx5e: Fix number of PFC counters reported to ethtool

Number of PFC counters used to count only number of priorities with PFC
enabled, but each priority has more than one counter, hence the need to
multiply it by the number of PFC counters per priority.

Fixes: cf678570

 ('net/mlx5e: Add per priority group to PPort 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>