PCI: endpoint: Set endpoint controller pointer to NULL

Set endpoint controller pointer to NULL in pci_epc_remove_epf()
to avoid -EBUSY on subsequent call to pci_epc_add_epf().

Add a check for NULL endpoint function pointer.

Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>