media: vivid: two unregistration fixes

When the media device registration fails, don't call media_device_unregister
since the device was never actually registered.

When removing the module also call media_device_cleanup() to avoid a memory leak.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>