pinctrl: sh-pfc: Make legacy function GPIO handling less fragile

If there are no function GPIOs, sh_pfc_register_gpiochip() returns early
with a success indicator.  This is fragile, as new code may be added
after the #ifdef block, which won't be executed in case of early return.

Invert the logic, so the code always continues until the end of the
function on success.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20191113101809.28600-1-geert+renesas@glider.be