IB/mlx5: Fix congestion counters in LAG mode

Congestion counters are counted and queried per physical function.
When working in LAG mode, CNP packets can be sent or received on both
of the functions, thus congestion counters should be aggregated from
the two physical functions.

Fixes: e1f24a79

 ("IB/mlx5: Support congestion related counters")
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>