PM / devfreq: exynos: dev_pm_opp_put_*() accepts NULL argument

The dev_pm_opp_put_*() APIs now accepts a NULL opp_table pointer and so
there is no need for us to carry the extra check. Drop them.

Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>