virtio-snd: check AUD_register_card return value

virtio-snd: check AUD_register_card return value

AUD_register_card might fail. Even though errp was passed as an
argument, the call's return value was not checked for failure.

Fixes: Coverity CID 1523899
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-Id: <20231109162034.2108018-1-manos.pitsidianakis@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>