media: v4l2-core: zero field base in struct v4l2_framebuffer

Make sure this field is always 0 since destructive overlays
are no longer supported.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>