usb: r8a66597-hcd: fix removed from an attached hub

fix the problem that when a USB hub is attached to the r8a66597-hcd and
a device is removed from that hub, it's likely that a kernel panic follows.

Reported-by: Markus Pietrek <Markus.Pietrek@emtrion.de>
Signed-off-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>