tape: beyond ARRAY_SIZE of viocd_diskinfo

Do not go beyond ARRAY_SIZE of tape_device and viotape_unitinfo

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>