hw/arm_sysctl: Clear sysctl cfgctrl start bit

The start bit should only be set to indicate that a function call is
underway, right now.  When done with function, clear it.
Signed-off-by: NChristoffer Dall <c.dall@virtualopensystems.com>
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/arm_sysctl.c

@@ -334,6 +334,7 @@ static void arm_sysctl_write(void *opaque, hwaddr offset,
         default:
             s->sys_cfgstat |= 2;        /* error */
         }
+        s->sys_cfgctrl &= ~(1 << 31);
         return;
     case 0xa8: /* SYS_CFGSTAT */
         if (board_id(s) != BOARD_ID_VEXPRESS) {