V4L/DVB (3192): Fix bttv sub-device unregister

- Fixes sub-device release for BTTV. Without this, DVB modules can't be reloaded

Signed-off-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@brturbo.com.br>