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

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

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

drivers/usb/class/cdc-wdm.c

@@ -548,7 +548,7 @@ static unsigned int wdm_poll(struct file *file, struct poll_table_struct *wait)
 
 	spin_lock_irqsave(&desc->iuspin, flags);
 	if (test_bit(WDM_DISCONNECTING, &desc->flags)) {
-		mask = POLLERR;
+		mask = POLLHUP | POLLERR;
 		spin_unlock_irqrestore(&desc->iuspin, flags);
 		goto desc_out;
 	}