PCI: endpoint: Finish virtual EP removal in pci_epf_remove_vepf()

PCI: endpoint: Finish virtual EP removal in pci_epf_remove_vepf()

When removing a virtual Endpoint, pci_epf_remove_vepf() failed to clear
epf_vf->epf_pf, which caused a subsequent pci_epf_add_vepf() to incorrectly
return -EBUSY:

  pci_epf_add_vepf(epf_pf, epf_vf)      // add
  pci_epf_remove_vepf(epf_pf, epf_vf)   // remove
  pci_epf_add_vepf(epf_pf, epf_vf)      // add again, -EBUSY error

Fix by clearing epf_vf->epf_pf in pci_epf_remove_vepf().

Link: https://lore.kernel.org/r/20241210-pci-epc-core_fix-v3-3-4d86dd573e4b@quicinc.com
Fixes: 1cf362e907f3 ("PCI: endpoint: Add support to add virtual function in endpoint core")
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Cc: stable@vger.kernel.org