usb: chipidea: host: set host to be null after hcd is freed

Set ci->hcd and ci->otg.host to be null in host_stop since the
hcd already freed.
Signed-off-by: Li Jun <jun.li@freescale.com>
Signed-off-by: Peter Chen <peter.chen@freescale.com>

drivers/usb/chipidea/host.c

@@ -190,6 +190,8 @@ static void host_stop(struct ci_hdrc *ci)
 			(ci->platdata->flags & CI_HDRC_TURN_VBUS_EARLY_ON))
 				regulator_disable(ci->platdata->reg_vbus);
 	}
+	ci->hcd = NULL;
+	ci->otg.host = NULL;
 }