net/mlx4_core: destroy workqueue when driver fails to register

When driver registration fails, we need to clean up the resources allocated
before. mlx4_core missed destroying the workqueue allocated.

This patch destroys the workqueue when registration fails.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>