[media] cx23885-dvb: check if dvb_attach() succeded

Fix possible null dereference for Leadtek DTV 3200H
XC4000 tuner when no firmware file available.

Cc: stable@kernel.org
Signed-off-by: Miroslav Slugen <thunder.mmm@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>