V4L/DVB (9194): stk-webcam: fix crash on close after disconnect

This patch prevents stk-webcam from updating usb device information
once the camera has been removed. This prevents a crash that would
otherwise occur if the camera is disconnected while it is still in
use.

Signed-off-by: David Ellingsworth <david@identd.dyndns.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>