gpio: mcp23s08: Make of_device_id array const

Make of_device_id array const, because all OF functions
handle it as const.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-mcp23s08.c

@@ -714,7 +714,7 @@ fail:
 
 #ifdef CONFIG_OF
 #ifdef CONFIG_SPI_MASTER
-static struct of_device_id mcp23s08_spi_of_match[] = {
+static const struct of_device_id mcp23s08_spi_of_match[] = {
 	{
 		.compatible = "microchip,mcp23s08",
 		.data = (void *) MCP_TYPE_S08,
@@ -738,7 +738,7 @@ MODULE_DEVICE_TABLE(of, mcp23s08_spi_of_match);
 #endif
 
 #if IS_ENABLED(CONFIG_I2C)
-static struct of_device_id mcp23s08_i2c_of_match[] = {
+static const struct of_device_id mcp23s08_i2c_of_match[] = {
 	{
 		.compatible = "microchip,mcp23008",
 		.data = (void *) MCP_TYPE_008,