vtuner: do not start a pes filter untill feeds are requested

dvb-usb-v2 no longer supports feeds to be started while
the frontend is sleeping (adap->active_fe is set to -1).
Not only does it not allow this anymore, but trying to do so
seems to trigger bugs, when adap->active_fe is used as an
index in adap->fe[]

So we wait till the vtuner device requests feeds to be
started, by that time the frontend has been activated
and tuned.