serial: pl011: Fix build breakage with !CONFIG_DMA_ENGINE

Commit 479e9b94 ("serial: Refactor uart_flush_buffer() from
uart_close") refactored the uart_flush_buffer() in uart_close() into
those drivers that define a flush_buffer() method. However, in the
amba-pl011 driver configured without CONFIG_DMA_ENGINE, flush_buffer()
is a NULL method, so the direct call fails to compile.

Check and call the flush_buffer() method through the ops table instead.
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -1689,7 +1689,8 @@ static void pl011_shutdown(struct uart_port *port)
 			plat->exit();
 	}
 
-	pl011_dma_flush_buffer(port);
+	if (uap->port.ops->flush_buffer)
+		uap->port.ops->flush_buffer(port);
 }
 
 static void