V4L/DVB (8026): Avoids an OOPS if dev struct can't be successfully recovered

On some alsa versions, it seems that snd_pcm_substream_chip(substream)
is returning a NULL pointer. This causes an OOPS, as reported by:

https://bugs.launchpad.net/ubuntu/+source/linux-ubuntu-modules-2.6.24/+bug/212271
https://bugs.launchpad.net/ubuntu/+source/alsa-driver/+bug/212960



This patch avoids the OOPS by not letting and open() succeed.

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