audio: omitting audiodev= parameter is only deprecated

Unfortunately, changes introduced in af2041ed "audio: audiodev=
parameters no longer optional when -audiodev present" breaks backward
compatibility.  This patch changes the error into a deprecation warning.

Signed-off-by: Kővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: 02d4328c33455742d01e0b62395013e95293c3ba.1566847960.git.DirtY.iCE.hu@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>