irqchip: exynos: look up irq using irq_find_mapping

Since we want to move to using the linear IRQ domain in the
future, we cannot rely on the irq numbers to be contiguous
and need to look up the irq from the hwirq using the domain.

This also turns the bogus comparison with NR_IRQ into a
more meaningful check to see if the number has a valid mapping.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>