[media] dvb_usb_v2: implement power-management for suspend

Put device full sleep on suspend, wake-up it on resume and acquire
retune in order to return same television channel.

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