Revert "pinctrl: consumer: use correct retval for placeholder functions"

This reverts commit 5a7d2efd.

As per discussion on the mailing list, this is not the right
thing to do. NULL cookies are valid in the stubs.

Reported-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>