[media] mb86a20s: Always reset the frontend with set_frontend

Always init the frontend when set_frontend is called. The rationale
is: it was noticed that, on some devices, it fails to lock with a
different channel. It seems that some other registers need to be
restored to its initial state, when the channel changes.
As it is better to reset everything, even wasting a few more
miliseconds than to loose channel lock, let's change the logic
to always reset.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>