fix hw/slavio_intctl.c system read address mask

diff -p -u -r1.24 slavio_intctl.c
@@ -145,7 +145,7 @@ static uint32_t slavio_intctlm_mem_readl
     SLAVIO_INTCTLState *s = opaque;
     uint32_t saddr, ret;
 
-    saddr = (addr & INTCTLM_MAXADDR) >> 2;
+    saddr = (addr & INTCTLM_MASK) >> 2;
     switch (saddr) {
     case 0:
         ret = s->intregm_pending & ~MASTER_DISABLE;