um: irq: Fix LAST_IRQ usage in init_IRQ()

LAST_IRQ was used incorrectly in init_IRQ.
Commit 09ccf036 forgot to update the for loop.
Fix this.

Fixes: 49da7e64 ("High Performance UML Vector Network Driver")
Fixes: 09ccf036 ("um: Fix off by one error in IRQ enumeration")
Signed-off-by: Erel Geron <erelx.geron@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.co.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>