V4L/DVB (5158): Dvb-ttpci: Fixed unregistering the vbi device

Fixed unregistering the vbi device for cards without analog tuner.
Thanks to Marco Schluessler <marco@lordzodiac.de> for pointing out this bug.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>