V4L/DVB (7537): cx88/saa7134: Fix: avoid OOPS on module unload

If frontend is not attached, both cx88-dvb and saa7134-dvb don't
register DVB. However, dvb unregister were inconditionally called.

Due to that, an OOPS is generated.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>