[media] media: au0828 fix to clear enable/disable/change source handlers

Fix to clear enable/disable/change source handlers in the media device
when media device is unregistered in au0828_unregister_media_device().
When au0828 module is removed, snd-usb-audio shouldn't call the handlers.
Clearing will ensure snd-usb-audio won't call them once au0828 is removed.

[mchehab@osg.samsung.com: fix a compilation breakage]
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>