V4L/DVB (7279): ivtv: Add missing sg_init_table()

If a dma transfer is attempted for either yuv or framebuffer output, a missing
sg_init_table() call causes a kernel BUG in scatterlist.h if CONFIG_DEBUG_SG
is set.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>