serial: st-asc: Fix unused variable warning for non DT.

This patch fixes a below warning reported by kbuild test robot.

drivers/tty/serial/st-asc.c:727:28: warning: 'asc_match' defined but
 not used [-Wunused-variable]

The code used in DT case is now ifdefed under CONFIG_OF to fix this
warning.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Reported-by: Kbuild Test Robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/st-asc.c

@@ -724,12 +724,14 @@ static struct asc_port *asc_of_get_asc_port(struct platform_device *pdev)
 	return &asc_ports[id];
 }
 
+#ifdef CONFIG_OF
 static struct of_device_id asc_match[] = {
 	{ .compatible = "st,asc", },
 	{},
 };
 
 MODULE_DEVICE_TABLE(of, asc_match);
+#endif
 
 static int asc_serial_probe(struct platform_device *pdev)
 {