bus: ti-sysc: Fix getting optional clocks in clock_roles

We can have holes in clock_roles with interface clock missing for
example. Currently getting an optional clock will fail if there are
only a functional clock and an optional clock.

Fixes: 09dfe581

 ("bus: ti-sysc: Add handling for clkctrl opt clocks")
Signed-off-by: Tony Lindgren <tony@atomide.com>