[media] dvb-usb: fix streaming failure on channel change

This fixes a bug introduced by a previous changeset entitled,
[media] dvb-usb: refactor MFE code for individual streaming config per frontend

As reported by Antti Palosaari, This error is shown by VLC when channel changed:

[0x7f1bbc000cd0] dvb access error: DMXSetFilter: failed with -1 (Invalid argument)
[0x7f1bbc000cd0] dvb access error: DMXSetFilter failed
[0x7f1bbc32f910] main stream error: cannot pre fill buffer

After my own investigations, I've determined that this error case occurs when
the application stops streaming but leaves the frontend and dvr devices open.
A typical example of this usage would be a channel change operation while
watching live television. The error occurs when the application attempts to
stream after tuning to the new channel.

To prevent this error, don't set adap->active_fe to -1 unless the application
closes the device.

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