[media] gspca: Call sd_stop0 on disconnect

This is necessary to ensure that worker-threads accessing the device
are stopped before our disconnect handler returns.

This causes a problem with stream_off calling sd_stop0 a second time
when the device handle is closed. This is fixed by setting
gscpa_dev->streaming to 0 on disconnect.

Note that now stream_off will never be called on a disconnected device,
and the present check can thus be removed from stream_off.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>