net/mlx4_core: Use sink counter for the VF default as fallback

Some old PF drivers don't let VFs allocate counters, in that case, use
the sink counter so the VF can load and operate properly.

Fixes: 6de5f7f6 ('net/mlx4_core: Allocate default counter per port')
Reported-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>