pinctrl: imx: fix allocation result check

Fix code to check correct variable value.

Reported-by: Abel Vesa <abel.vesa@nxp.com>
Fixes: 02f11713

 ("pinctrl: imx: prepare for making "group_names" in "function_desc" const")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20211222212807.27122-1-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>