V4L/DVB (9087): gspca: Image transfer by bulk uses altsetting 0 with any buffer size.

- gspca_dev field 'bulk_size' added.
- when only one altsetting usable, do image transfer by bulk.

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