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

[ Upstream commit 7b4f8ac2f1acdff3c0cce23d8c3b86434a6e768a ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>