V4L/DVB: gspca - sonixj: Bad detection of the end of image

The 'end of image' block may be splitted between two ISOC packets.
This case was not tested, so, some images could be lost and concatenated
to previous one(s), raising 'frame overflow' errors.

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