opp: Allow _generic_set_opp_clk_only() to work for non-freq devices

In order to avoid conditional statements at the caller site, this patch
updates _generic_set_opp_clk_only() to work for devices that don't
change frequency (like power domains, etc.). Return 0 if the clk pointer
passed to this routine is not valid.

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