net/mlx4_core: Check before cleaning counters bitmap

If counters are not supported by the device. The indices bitmap table is not
allocated during initialization. Add the symmetrical check before cleaning
the counters bitmap table or freeing a counter.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>