media: dvbdev: Fix memory leak in dvb_media_device_free()

dvb_media_device_free() is leaking memory. Free `dvbdev->adapter->conn`
before setting it to NULL, as documented in include/media/media-device.h:
"The media_entity instance itself must be freed explicitly by the driver
if required."

Link: https://syzkaller.appspot.com/bug?id=9bbe4b842c98f0ed05c5eed77a226e9de33bf298

Link: https://lore.kernel.org/linux-media/20201211083039.521617-1-yepeilin.cs@gmail.com
Cc: stable@vger.kernel.org
Fixes: 0230d60e ("[media] dvbdev: Add RF connector if needed")
Reported-by: syzbot+7f09440acc069a0d38ac@syzkaller.appspotmail.com
Signed-off-by: Peilin Ye <yepeilin.cs@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/dvb-core/dvbdev.c

@@ -241,6 +241,7 @@ static void dvb_media_device_free(struct dvb_device *dvbdev)
 
 	if (dvbdev->adapter->conn) {
 		media_device_unregister_entity(dvbdev->adapter->conn);
+		kfree(dvbdev->adapter->conn);
 		dvbdev->adapter->conn = NULL;
 		kfree(dvbdev->adapter->conn_pads);
 		dvbdev->adapter->conn_pads = NULL;