PCI: endpoint: Add missing NULL check for alloc_workqueue()

In the Linux kernel, the following vulnerability has been resolved: PCI: endpoint: Add missing NULL check for alloc_workqueue() alloc_workqueue() can return NULL on memory allocation failure. Without proper error checking, this may lead to a NULL pointer dereference when queue_work() is later called with the NULL workqueue pointer in epf_ntb_epc_init(). Add a NULL check immediately after alloc_workqueue() and return -ENOMEM on failure to prevent the driver from loading with an invalid workqueue pointer.