gpio: sim: fix a typo in comment

It was supposed to say 'for' not 'fo'.

Signed-off-by: Bartosz Golaszewski <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>

drivers/gpio/gpio-sim.c

···
 	 * 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;