V4L/DVB: cx18: overhaul ALSA PCM device handling so it works

Add code so that the PCM ALSA device actually works, and update the
cx18-streams mechanism so that it passes the data off to the cx18-alsa module.

This work was sponsored by ONELAN Limited.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>