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.se


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>