gpio: mpc8xxx: 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 'irqn', so making 'irqn' an int.

Eliminate the following coccicheck warning:
./drivers/gpio/gpio-mpc8xxx.c:391:5-21: WARNING: Unsigned expression
compared with zero: mpc8xxx_gc -> irqn < 0

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: 0b39536c ("gpio: mpc8xxx: Fix IRQ check in mpc8xxx_probe")
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>