USB: cdc-acm: do not update PM busy on read errors

There's no need to update the runtime PM last_busy field on read urb
errors (e.g. when the urb is being killed on shutdown).

Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/cdc-acm.c

···
 		dev_dbg(&acm->data->dev, "%s - disconnected\n", __func__);
 		return;
 	}
-	usb_mark_last_busy(acm->dev);
 
 	if (urb->status) {
 		dev_dbg(&acm->data->dev, "%s - non-zero urb status: %d\n",
 							__func__, urb->status);
 		return;
 	}
+
+	usb_mark_last_busy(acm->dev);
+
 	acm_process_read_urb(acm, urb);
 
 	/* throttle device if requested by tty */