usb: core: phy: add the SPDX-License-Identifier and include guard

This clarifies the license of the code. While here also add an include
guard to the header file.

Fixes: 07dbff0d ("usb: core: add a wrapper for the USB PHYs on the HCD")
Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/phy.h

+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * USB roothub wrapper
+ *
+ * Copyright (C) 2018 Martin Blumenstingl <martin.blumenstingl@googlemail.com>
+ */
+
+#ifndef __USB_CORE_PHY_H_
+#define __USB_CORE_PHY_H_
+
 struct device;
 struct usb_phy_roothub;
 
@@ -13,3 +23,5 @@ int usb_phy_roothub_suspend(struct device *controller_dev,
 			    struct usb_phy_roothub *phy_roothub);
 int usb_phy_roothub_resume(struct device *controller_dev,
 			   struct usb_phy_roothub *phy_roothub);
+
+#endif /* __USB_CORE_PHY_H_ */