opp: Don't ignore clk_get() errors other than -ENOENT

Not all devices that need to use OPP core need to have clocks, a missing
clock is fine in which case -ENOENT shall be returned by clk_get().

Anything else is an error and must be handled properly.

Reported-by: Dmitry Osipenko <digetx@gmail.com>
Tested-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>