usb: isp1760: Don't expose hcd to glue code from isp1760_register

The glue code probe functions don't need to access the hcd structure
anymore. Modify isp1760_register to return an integer error code instead
of the hcd pointer.

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