docs: pin-control: Fix error path for control state example

The error is constructed using the wrong variable.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20210328164222.720525-1-niklas.soderlund+renesas@ragnatech.seSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/pin-control.rst

@@ -1235,7 +1235,7 @@ default state like this::
 		foo->s = pinctrl_lookup_state(foo->p, PINCTRL_STATE_DEFAULT);
 		if (IS_ERR(foo->s)) {
 			/* FIXME: clean up "foo" here */
-			return PTR_ERR(s);
+			return PTR_ERR(foo->s);
 		}
 
 		ret = pinctrl_select_state(foo->s);