net/mlx4_core: Fix potential corruption in counters database

The error flow in procedure handle_existing_counter() is wrong.

The procedure should exit after encountering the error, not continue
as if everything is OK.

Fixes: 68230242

 ('net/mlx4_core: Add port attribute when tracking counters')
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>