media: mc: Clear minor number reservation at unregistration time

Clear the media device's minor number reservation at unregister time as
there's no need to keep it reserved for longer. This makes it possible to
reserve the same minor right after unregistration.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hverkuil+cisco@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>