gpio: mcp23s08: make driver depend on OF_GPIO

The MCP23S08 driver certainly accesses fields inside the
struct gpio_chip that are only available under CONFIG_OF_GPIO
not just CONFIG_OF, so update the Kconfig and driver to reflect
this.

Cc: Alexander Stein <alexander.stein@systec-electronic.com>
Cc: Phil Reid <preid@electromag.com.au>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -1131,6 +1131,7 @@ menu "SPI or I2C GPIO expanders"
 
 config GPIO_MCP23S08
 	tristate "Microchip MCP23xxx I/O expander"
+	depends on OF_GPIO
 	select GPIOLIB_IRQCHIP
 	help
 	  SPI/I2C driver for Microchip MCP23S08/MCP23S17/MCP23008/MCP23017

drivers/gpio/gpio-mcp23s08.c

@@ -564,7 +564,7 @@ static int mcp23s08_probe_one(struct mcp23s08 *mcp, struct device *dev,
 	mcp->chip.direction_output = mcp23s08_direction_output;
 	mcp->chip.set = mcp23s08_set;
 	mcp->chip.dbg_show = mcp23s08_dbg_show;
-#ifdef CONFIG_OF
+#ifdef CONFIG_OF_GPIO
 	mcp->chip.of_gpio_n_cells = 2;
 	mcp->chip.of_node = dev->of_node;
 #endif