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.

Reported-by: Wang, Yalin <Yalin.Wang@sonymobile.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>