V4L/DVB (10756): cx18: Slim down instance handling, build names from v4l2_device.name

Convert card instance handling to a lighter weight mechanism like ivtv.
Also convert name strings and debug messages to use v4l2_device.name.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>