pinctrl: axp209: Fix NULL pointer dereference after allocation

axp20x_build_funcs_groups allocates groups via devm_kcalloc and tries to
dereference without checking for NULL. This patch avoids such a
scenario.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>