[POWERPC] viopath: Use completion

Use a completion instead of abusing a semaphore for hypervisor event
completion in viopath.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>