powerpc/pseries: Cleanup use of notifier_from_errno()

Minor cleanup of notifier_from_errno() in powerpc.

notifier_from_errno() now contains the if(ret)/else conditional.
There is no need to do it in the powerpc code.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>