USB: serial: symbolserial: use irqsave() in USB's complete callback

The USB completion callback does not disable interrupts while acquiring
the lock. We want to remove the local_irq_disable() invocation from
__usb_hcd_giveback_urb() and therefore it is required for the callback
handler to disable the interrupts while acquiring the lock.
The callback may be invoked either in IRQ or BH context depending on the
USB host controller.
Use the _irqsave() variant of the locking primitives.

Signed-off-by: John Ogness <[e-mail]>
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Johan Hovold <[e-mail]>

drivers/usb/serial/symbolserial.c

···
 	struct symbol_private *priv = usb_get_serial_port_data(port);
 	unsigned char *data = urb->transfer_buffer;
 	int status = urb->status;
+	unsigned long flags;
 	int result;
 	int data_length;
 
···
 	}
 
 exit:
-	spin_lock(&priv->lock);
+	spin_lock_irqsave(&priv->lock, flags);
 
 	/* Continue trying to always read if we should */
 	if (!priv->throttled) {
···
 							__func__, result);
 	} else
 		priv->actually_throttled = true;
-	spin_unlock(&priv->lock);
+	spin_unlock_irqrestore(&priv->lock, flags);
 }
 
 static int symbol_open(struct tty_struct *tty, struct usb_serial_port *port)