usb: chipidea: host: Only disable the vbus regulator if it is not NULL

Commit 40ed51a4 (usb: chipidea: host: add vbus regulator
control) introduced a smatch complaint because regulator_disable() is called
without checking whether ci->platdata->reg_vbus is not NULL.

Fix this by adding the check.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/chipidea/host.c

@@ -88,7 +88,8 @@ static int host_start(struct ci_hdrc *ci)
 	return ret;
 
 disable_reg:
-	regulator_disable(ci->platdata->reg_vbus);
+	if (ci->platdata->reg_vbus)
+		regulator_disable(ci->platdata->reg_vbus);
 
 put_hcd:
 	usb_put_hcd(hcd);