V4L/DVB (4838): Fix cx88-blackbird null pointer

Allows 'debug=1' for cx88-blackbird module (dev needs to be valid for
dprintk). Fixes a null-pointer dereference when using debug=1.

Signed-off-by: Jelle Foks <jelle@foks.8m.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>