V4L/DVB (13866): gspca - main: Add the cam flag 'no_urb_create'.

This flag permits subdrivers to create specific transfer URBs.

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