usb: isp1760: Decouple usb_hdc and isp1760_priv

Allocate the driver private data structure manually instead of using the
usb_hcd private space. This will allow skipping hcd registration for the
isp1761 when used in device mode only.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>