pinctrl: constify pinmux_generic_get_function()

pinctrl: constify pinmux_generic_get_function()

With all users of struct function_desc limited to only accessing it using
the dedicated function and never modifying it, we can now constify the
return value of pinmux_generic_get_function() treewide.

Reviewed-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be> # renesas
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be> # renesas
Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>