hw/gpio: npcm7xx: fixup out-of-bounds access

hw/gpio: npcm7xx: fixup out-of-bounds access

The reg isn't validated to be a possible register before
it's dereferenced for one case.  The mmio space registered
for the gpio device is 4KiB but there aren't that many
registers in the struct.

Cc: qemu-stable@nongnu.org
Fixes: 526dbbe0874 ("hw/gpio: Add GPIO model for Nuvoton NPCM7xx")
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250226024603.493148-1-venture@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>