[media] dvb-usb-v2: postpone removal of media_device

We should not remove the media_device until its last usage,
or we may have use after free troubles.

So, move the per-adapter media_device removal to happen at
the end of the adapter removal code.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>