V4L/DVB (8232): gspca: Change the USERPTR mechanism.

main:     Change the packet copy mechanism for userptr.
          Cannot do reqbufs ioctl when already done and count != 0.
          Accept count < frame size in read().

Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>