V4L/DVB (10125): em28xx: Don't do AC97 vendor detection for i2s audio devices

The current code was trying to query the AC97 registers for the vendor
information even if it was clearly not a AC97 audio device (resulting in errors
in the dmesg output).  This was due to a bug in the way we did the check.

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