usb: core: phy: add missing forward declaration for "struct device"

Currently hcd.c is the only consumer of the usb_phy_roothub logic. This
already includes the required header files so struct device is known.
However, future consumers might not know about struct device.
Add a forward declaration for struct device to fix potential future
consumers which don't include any of the struct device API headers.

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>