V4L/DVB (11710): gspca - main: Webcams cannot do both isoc and bulk image transfers.

Let the subdrivers to set the 'image transfer by bulk' flag.

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