USB: cdc-wdm: poll must return POLLHUP if device is gone

commit 616b6937 upstream.

Else the poll will be restarted indefinitely in a tight loop,
preventing final device cleanup.

Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>