mfd: sm501: Use GPIO_LOOKUP_IDX() helper macro

i801_add_mux() fills in the GPIO lookup table by manually populating an
array of gpiod_lookup structures.  Use the existing GPIO_LOOKUP_IDX()
helper macro instead, to relax a dependency on the gpiod_lookup
structure's member names.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200511145257.22970-3-geert+renesas@glider.be


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