dca: check against empty dca_domains list before unregister provider

When providers get blocked unregister_dca_providers() is called ending up with dca_providers and dca_domain lists emptied. Dca should be prevented from trying to unregister any provider if dca_domain list is found empty.

---

 drivers/dca/dca-core.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

index bc6f5fa..819dfda 100644
@@ -420,6 +420,11 @@ void unregister_dca_provider(struct dca_
 
 	raw_spin_lock_irqsave(&dca_lock, flags);
 
+	if (list_empty(&dca_domains)) {
+		raw_spin_unlock_irqrestore(&dca_lock, flags);
+		return;
+	}
+
 	list_del(&dca->node);
 
 	pci_rc = dca_pci_rc_from_dev(dev);