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.