[media] dvb_usb_v2: do not check active fe when stop streaming

Do not check active frontend as it could be already shutdown.
Add some more debugs.

It turns out that device is put sleep earlier than streaming
is stopped in some cases. That is because streaming is running
different task and there is no locking.

Maybe some locking could be good idea to force stop streaming
before device is shut down. I can guess there could be problems
in someday cases like chip is sleep but streaming control is
requested after that...

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