V4L/DVB (10385): gspca - main: Fix memory leak when USB disconnection while streaming.

Resetting the streaming flag on disconnection prevented the URBs to be freed
when streaming was active.
Also, USBs cannot be killed after disconnection (oops in [usbcore] unlink1).

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