[media] dvb_usb_v2: call streaming_ctrl() before kill urbs

Logically it is better ask hardware to stop streaming before
killing urbs carrying stream. Earlier it was just opposite.
Now code runs:
* submit urbs
* start streaming
** streaming ongoing **
* stop streaming
* kill urbs

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>