PCI: endpoint: Use after free in pci_epf_unregister_driver()

We need to use list_for_each_entry_safe() because the
pci_ep_cfs_remove_epf_group() function frees "group".

Fixes: ef1433f7 ("PCI: endpoint: Create configfs entry for each pci_epf_device_id table entry")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>