usb: xhci-rcar: deregister before deactivating clock

During the execution of xhci_deregister xHCI registers are accessed. If
the clock is already deactivated when xhci_deregister is called this can
lead to undefined behavior. Change the order to deregister the device
before deactivating the clock.

Signed-off-by: Matthias Blankertz <matthias.blankertz@cetitec.com>