pinctrl: imx: fix assigning groups names

This fixes regression caused by incorrect array indexing.

Reported-by: Fabio Estevam <festevam@gmail.com>
Fixes: 02f11713 ("pinctrl: imx: prepare for making "group_names" in "function_desc" const")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Tested-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20211227122237.6363-1-zajec5@gmail.com


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