um: Fix off by one error in IRQ enumeration

Fix an off-by-one in IRQ enumeration

Fixes: 49da7e64f3 ("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>

arch/um/include/asm/irq.h

@@ -23,7 +23,7 @@
 #define VECTOR_BASE_IRQ		15
 #define VECTOR_IRQ_SPACE	8
 
-#define LAST_IRQ (VECTOR_IRQ_SPACE + VECTOR_BASE_IRQ)
+#define LAST_IRQ (VECTOR_IRQ_SPACE + VECTOR_BASE_IRQ - 1)
 
 #else