pinctrl: meson: fix incorrect usage of ENOSYS

ENOSYS is special and should only be used for incorrect syscall number.
It is not the case here. let's use ENOTSUPP instead.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>