extcon: Fix the checkpatch warning

This patch fixes the checkpatch warning about coding style.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
diff --git a/drivers/extcon/extcon-usb-gpio.c b/drivers/extcon/extcon-usb-gpio.c
index 3f0bad3..de67fce 100644
--- a/drivers/extcon/extcon-usb-gpio.c
+++ b/drivers/extcon/extcon-usb-gpio.c
@@ -214,7 +214,7 @@
 static SIMPLE_DEV_PM_OPS(usb_extcon_pm_ops,
 			 usb_extcon_suspend, usb_extcon_resume);
 
-static struct of_device_id usb_extcon_dt_match[] = {
+static const struct of_device_id usb_extcon_dt_match[] = {
 	{ .compatible = "linux,extcon-usb-gpio", },
 	{ /* sentinel */ }
 };