um: Fix off by one error in IRQ enumeration

Fix an off-by-one in IRQ enumeration

Fixes: 49da7e64 ("High Performance UML Vector Network Driver")
Reported by: Dana Johnson <djohns042@gmail.com>
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>