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

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.

Fixes: e35f56bb0330 ("PCI: endpoint: Support NTB transfer between RC and EP")
Fixes: 8b821cf76150 ("PCI: endpoint: Add EP function driver to provide NTB functionality")
Signed-off-by: Haotian Zhang <vulab@iscas.ac.cn>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251110040446.2065-1-vulab@iscas.ac.cn