powerpc/eeh: Rework pci_dev lookup in debugfs attributes

Pull the string -> pci_dev lookup stuff into a helper function. No functional change.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201103051512.919333-1-oohall@gmail.com