arm_mptimer: Fix timer shutdown and mode change

The running timer can't be stopped because timer control code just
doesn't handle disabling the timer. Fix it by deleting the timer if
the enable bit is cleared.

The timer won't start periodic ticking if a ONE-SHOT -> PERIODIC mode
change happens after a one-shot tick was completed. Fix it by
re-starting ticking if the timer isn't ticking right now.

To avoid code churning, these two fixes are squashed in one commit.
Signed-off-by: NDmitry Osipenko <digetx@gmail.com>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/timer/arm_mptimer.c

@@ -122,11 +122,18 @@ static void timerblock_write(void *opaque, hwaddr addr,
     case 8: /* Control.  */
         old = tb->control;
         tb->control = value;
-        if (((old & 1) == 0) && (value & 1)) {
-            if (tb->count == 0 && (tb->control & 2)) {
+        if (value & 1) {
+            if ((old & 1) && (tb->count != 0)) {
+                /* Do nothing if timer is ticking right now.  */
+                break;
+            }
+            if (tb->control & 2) {
                 tb->count = tb->load;
             }
             timerblock_reload(tb, 1);
+        } else if (old & 1) {
+            /* Shutdown the timer.  */
+            timer_del(tb->timer);
         }
         break;
     case 12: /* Interrupt status.  */