usb: isp1760: Initialize the bus interface in core code

Although the corresponding register is part of the HCD register space,
processor bus initialization is not specific to the HCD. To prepare for
device controller support, move bus interface initialization to core
code.

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