cxgb4: remove workqueue when driver registration fails

When driver registration fails, we need to clean up the resources allocated
before. cxgb4 missed to destroy the workqueue allocated at the very beginning.

This patch destroies the workqueue when registration fails.

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