powerpc/iseries: Initialise on-stack completion

get_viotape_info() declares a vio_waitevent on the stack, which
contains a completion, but never initialises the completion.

I have no idea how this ever worked, and on recent kernels it causes
an oops in handle_tape_event() when we access the non-initialised
completion.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>