sunxi: A64: OHCI: prevent turning off shared USB clock

On the A64 the clock for the first USB controller is actually the parent
of the clock for the second controller, so turning them off in that order
makes the system hang.
Fix this by only turning off *both* clocks when the *last* OHCI controller
is brought down. This covers the case when only one controller is used.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>