EEH core pci_dn de-lousing

Removes most of the uses of pci_dn in the EEH core. There's a few
stragglers remaining in pseries specific bits of kernel/eeh*.c mainly
the the support for "open sriov" where the hypervisor allows the guest
to manage SR-IOV physical functions. We can largely ignore that on
non-pseries platforms though.

There'll be a follow up to this which actually removes the use of pci_dn
from PowerNV entirely and we can start looking at properly supporting
native PCIe. At last.

Oliver