usb: r8a66597-hcd: fix wrong data access in SuperH on-chip USB

When I used SuperH on-chip USB, there was the problem that accessed
r8a66597_root_hub which was not allocated.

Signed-off-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>