[media] tm6000-dvb: Fix module unload

dvb_unregister_frontend has to be called before detach. Otherwise the
unregister call will segfault. This made tm6000-dvb module unload unusable.

Signed-off-by: Julian Scheel <julian@jusst.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>