PCI: endpoint: Fix a potential NULL pointer dereference

In case alloc_workqueue() fails, return -ENOMEM to avoid
potential NULL pointer dereferences.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
[lorenzo.pieralisi@arm.com: commit log and code update]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>