gpio: realtek-otto: fix GPIO line IRQ offset

The irqchip uses one domain for all GPIO lines, so the line offset
should be determined w.r.t. the first line of the first port, not the
first line of the triggered port.

Fixes: 0d82fb11 ("gpio: Add Realtek Otto GPIO support")
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>