powerpc/eeh: Cleanup logic in eeh_rmv_from_parent_pe()

Move the call to eeh_dev_to_pe() up, so that later it's clear that
"pe" isn't NULL.

Signed-off-by: Sam Bobroff <sbobroff@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>