pinctrl: renesas: rza2: Fix potential NULL pointer dereference

`chip.label` in rza2_gpio_register() could be NULL.
Add the missing check.

Signed-off-by: Chenyuan Yang <chenyuan0y@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Link: https://lore.kernel.org/20250210232552.1545887-1-chenyuan0y@gmail.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>