pinctrl: axp209: Fix NULL pointer dereference after allocation

There is no check that allocation in axp20x_funcs_groups_from_mask
is successful.
The patch adds corresponding check and return values.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Anton Vasilyev <vasilyev@ispras.ru>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>