audio: use object link instead of qdev property to pass wm8750 reference

audio: use object link instead of qdev property to pass wm8750 reference

According to qdev-properties.h, properties of pointer type should
be avoided, it seems a link type property is a good substitution.

Cc: Jan Kiszka <jan.kiszka@web.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Mao Zhongyi <maozhongyi@cmss.chinamobile.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181022074050.19638-3-maozhongyi@cmss.chinamobile.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>