pinctrl: pinmux: Fix kerneldoc for pinmux_generic_add_function()

Correct the incorrect function name and description.

Fixes: a76edc89

 ("pinctrl: core: Add generic pinctrl functions for managing groups")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>