V4L/DVB: cx18: rework cx18-alsa module loading to support automatic loading

Restructure the way the module gets loaded so that it gets loaded automatically
when cx18 is loaded, and make it work properly if there are multiple cards
present (since the old code would only take one opportunity to connect to cx18
instances when the module first loaded).

This work was sponsored by ONELAN Limited.

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