V4L/DVB: gspca - main: Simplify image building

The image pointer and its length are now in the main structure instead
of in the frame buffer. They are updated on application vidioc_qbuf and
in the URB interrupt function when ending an image.

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>