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>