powerpc/pseries: Avoid spurious error during hotplug CPU add

During hotplug CPU add we get the following error:

Unexpected Error (0) returned from configure-connector

ibm,configure-connector returns 0 for configuration complete, so
catch this and avoid the error.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: <stable@kernel.org>