docs/pinctrl: fix compile errors in example code

The code in the example does not build for a few trivial errors: type
mismatch in callback, missing semicolon. Fix them to help newcomers using
the example as a starting point.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Link: https://lore.kernel.org/r/20190705143043.1929-1-luca@lucaceresoli.net


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