Sparc32: fix free-run timer

According to Sun4M System Architecture Manual chapter 5.3.2, a limit
of 0 will not generate interrupts.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/slavio_timer.c

@@ -128,7 +128,8 @@ static void slavio_timer_irq(void *opaque)
     slavio_timer_get_out(t);
     DPRINTF("callback: count %x%08x\n", t->counthigh, t->count);
     t->reached = TIMER_REACHED;
-    if (!slavio_timer_is_user(tc)) {
+    /* there is no interrupt if user timer or free-run */
+    if (!slavio_timer_is_user(tc) && t->limit != 0) {
         qemu_irq_raise(t->irq);
     }
 }