powerpc/pseries/eeh: Clean up pe_config_addr lookups

De-duplicate, and fix up the comments, and make the prototype just take a
pci_dn since the job of the function is to return the pe_config_addr of the
PE which contains a given device.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200918093050.37344-6-oohall@gmail.com