gpiolib: irqchip: use irq_find_mapping while removing irqchip
There is no guarantee that VIRQs will be allocated sequentially
for gpio irqchip in gpiochip_irqchip_add().
Therefore, it's unsafe to dispose VIRQ in gpiochip_irqchip_remove()
basing on index relatively to stored irq_base value.
Hence, use irq_find_mapping for VIRQ finding in gpiochip_irqchip_remove()
instead of irq_base + index.