opp: No need to check clk for errors

Clock is not optional for users who call into dev_pm_opp_set_rate().
Remove the unnecessary checks.

While at it also drop the local variable for clk and use opp_table->clk
instead.

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