mfd: axp20x: Constify axp20x_acpi_match and rid unused warning

axp20x.c:239:30:
  warning: ‘axp20x_acpi_match’ defined but not used [-Wunused-variable]
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/axp20x.c

@@ -236,7 +236,7 @@ static const struct i2c_device_id axp20x_i2c_id[] = {
 };
 MODULE_DEVICE_TABLE(i2c, axp20x_i2c_id);
 
-static struct acpi_device_id axp20x_acpi_match[] = {
+static const struct acpi_device_id axp20x_acpi_match[] = {
 	{
 		.id = "INT33F4",
 		.driver_data = AXP288_ID,