net/mlx4_core: Fix unintialized variable used in error path

The uninitialized value name in mlx4_en_activate_cq was used in order
to print an error message. Fixing it by replacing it with cq->vector.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Carol L Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>