media: em28xx: add missing em28xx_close_extension

If em28xx dev has ->dev_next pointer, we need to delete ->dev_next list
node from em28xx_extension_devlist on disconnect to avoid UAF bugs and
corrupted list bugs, since driver frees this pointer on disconnect.

Reported-and-tested-by:  <syzbot+a6969ef522a36d3344c9@syzkaller.appspotmail.com>

Fixes: 1a23f81b

 ("V4L/DVB (9979): em28xx: move usb probe code to a proper place")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>