devm_kasprintf() can return a NULL pointer on failure but this returned
value is not checked. Fix this lack and check the returned value.

Found by code review.

Cc: stable@vger.kernel.org
Fixes: a0f160ff ("pinctrl: add pinctrl/GPIO driver for Apple SoCs")
Signed-off-by: Ma Ke <make24@iscas.ac.cn>
Reviewed-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/20240905020917.356534-1-make24@iscas.ac.cnSigned-off-by: Linus Walleij <linus.walleij@linaro.org>