cdc-wdm: Don't clear WDM_READ unless entire read buffer is emptied

The WDM_READ flag is cleared later iff desc->length is reduced to 0.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-wdm.c

@@ -455,7 +455,6 @@ static ssize_t wdm_read
 			spin_unlock_irq(&desc->iuspin);
 			goto retry;
 		}
-		clear_bit(WDM_READ, &desc->flags);
 		cntr = desc->length;
 		spin_unlock_irq(&desc->iuspin);
 	}