um: irq: don't set the chip for all irqs

Setting a chip for an interrupt marks it as allocated. Since UM doesn't
support dynamic interrupt numbers (yet), it means we cannot simply
increase NR_IRQS and then use the free irqs between LAST_IRQ and NR_IRQS
with gpio-mockup or iio testing drivers as irq_alloc_descs() will fail
after not being able to neither find an unallocated range of interrupts
nor expand the range.

Only call irq_set_chip_and_handler() for irqs until LAST_IRQ.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>