Fix a potential issue in mpc52xx uart driver

mpc52xx_uart_int and __uart_put_char both try to acquire the
port->lock. Therefore the function sequence of:

mpc52xx_uart_int--> ...-->flush_to_ldisc-->...-->__uart_put_char

can potentially trigger a deadlock. To avoid this deadlock a fix
similar to that found in the 8250.c serial driver is applied. The
deadlock is avoided by releasing the lock before pushing a buffer
and reacquiring it when completed.
Signed-off-by: NAndrew Liu <shengping.liu@windriver.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

drivers/serial/mpc52xx_uart.c

@@ -783,7 +783,9 @@ mpc52xx_uart_int_rx_chars(struct uart_port *port)
 		}
 	}
 
+	spin_unlock(&port->lock);
 	tty_flip_buffer_push(tty);
+	spin_lock(&port->lock);
 
 	return psc_ops->raw_rx_rdy(port);
 }