serial-uartlite: fix missing locking in isr

The uartlite driver suffers from missing/duplicate/corrupted character
data when the interrupt handler runs concurrently with access to the
device from another cpu. Take the port spinlock to exclude concurrent
access.

Signed-off-by: Rich Felker <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/serial/uartlite.c

···
 {
 	struct uart_port *port = dev_id;
 	int busy, n = 0;
+	unsigned long flags;
 
+	spin_lock_irqsave(&port->lock, flags);
 	do {
 		int stat = uart_in32(ULITE_STATUS, port);
 		busy  = ulite_receive(port, stat);
 		busy |= ulite_transmit(port, stat);
 		n++;
 	} while (busy);
+
+	spin_unlock_irqrestore(&port->lock, flags);
 
 	/* work done? */
 	if (n > 1) {