usb_wwan: fix runtime PM in error case

An error in the write code path would permanently disable
runtime PM in this driver

Signed-off-by: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/usb/serial/usb_wwan.c b/drivers/usb/serial/usb_wwan.c
index 7bd0685..07cdc6c 100644
--- a/drivers/usb/serial/usb_wwan.c
+++ b/drivers/usb/serial/usb_wwan.c
@@ -261,6 +261,7 @@
 				intfdata->in_flight--;
 				spin_unlock_irqrestore(&intfdata->susp_lock,
 						       flags);
+				usb_autopm_put_interface_async(port->serial->interface);
 				continue;
 			}
 		}