usb: isp1760: Unmap I/O registers at platform device removal

The I/O registers are mapped in the HCD code but must be unmapped by
glue code. The OF and PCI glue code unmap them correctly but the
platform glue code is missing an iounmap() call. Fix it.

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