V4L/DVB (9114): dib0700: fix bad assignment of dib0700_xc5000_tuner_callback after return call

Put callback definition before function return and cleanup callback:

Fix a bug where the xc5000 callback was being set *after* the
return call (essentially resulting in dead code).

Also cleanup the callback function to detect unknown commands.

Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>