V4L/DVB: gspca - JPEG subdrivers: Don't allocate the JPEG header

The JPEG header is now included in the subdriver structure instead
of being allocated and freed at capture start and stop.

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