powerpc/eeh: Slightly simplify eeh_add_to_parent_pe()

Simplify some needlessly complicated boolean logic in
eeh_add_to_parent_pe().

Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/09259a50308f10aa764695912bc87dc1d1cf654c.1565930772.git.sbobroff@linux.ibm.com