V4L/DVB (11979): em28xx: don't create audio device if not supported

In cases where the device does not actually provide a USB audio class *or*
vendor audio, do not load the driver that provides vendor audio support (such
as the KWorld 2800d).  Otherwise, the /dev/audio1 device file gets created and
users get confused.

Also, reworks the logic a bit so that we don't try to inspect the register
content if the register read failed entirely.

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