bus: ti-sysc: Fix sysc_unprepare() when no clocks have been allocated

If we return early before ddata->clocks have been allocated we will get a
NULL pointer dereference in sysc_unprepare(). Let's fix this by returning
early when no clocks are allocated.

Fixes: 0eecc636

 ("bus: ti-sysc: Add minimal TI sysc interconnect target driver")
Signed-off-by: Tony Lindgren <tony@atomide.com>