gpio: idt3243x: Fix an ignored error return from platform_get_irq()

The return from the call to platform_get_irq() is int, it can be
a negative error code, however this is being assigned to an unsigned
int variable 'parent_irq', so making 'parent_irq' an int.

Eliminate the following coccicheck warning:
./drivers/gpio/gpio-idt3243x.c:167:6-16: WARNING: Unsigned expression
compared with zero: parent_irq < 0

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 30fee1d7 ("gpio: idt3243x: Fix IRQ check in idt_gpio_probe")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>