[media] media: au0828 handle media_init and media_register window

Media device initialization and registration steps are split. There is a
window between media device init and media device register during usb
probe.

au0828 bridge driver and snd-usb-audio could try to initialize the media
device, if they simply checked, whether the device has been registered.
They also need to check whether the device has been initialized.

Change the au0828-core to check if media device is already initialized
during initialization step and check if media device is already
registered during the registration step.

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