[POWERPC] properly declare onstack completion in iseries veth

The iSeries veth driver uses an on-stack struct completion that
it initializes using the COMPLETION_INITIALIZER instead of
COMPLETION_INITIALIZER_ONSTACK macro, causing problems with
lockdep.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>