gpio: sim: fix a typo in comment

It was supposed to say 'for' not 'fo'.
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpio-sim.c

@@ -488,7 +488,7 @@ struct gpio_sim_device {
 	 * This structure however can be modified by callbacks of different
 	 * attributes so we need another lock.
 	 *
-	 * We use this lock fo protecting all data structures owned by this
+	 * We use this lock for protecting all data structures owned by this
 	 * object too.
 	 */
 	struct mutex lock;