V4L/DVB: pvrusb2: Fix kernel oops at device unregistration

pvrusb2: Fix oops caused by touching deleted memory after
unregistration.  This bug was introduced when we had started using
video_device_node_name() - that function was being called potentially
after the underlying structure (referenced by that function) had been
deleted.  The fix rearranges things slightly so that the function is
called before destruction takes place.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>