powerpc/powernv: Drop phb->bdfn_to_pe()

The last usage of pnv_phb::bdfn_to_pe() was removed in
ff57b454

 ("powerpc/eeh: Do probe on pci_dn"), so drop it.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>