PM / devfreq: tegra: Set freq in rate callback

As per the documentation of the devfreq_dev_profile.target callback, set
the freq argument to the new frequency before returning.

This caused endless messages like this after recent changes in the core:

devfreq 6000c800.actmon: Couldn't update frequency transition information.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reported-by: Tyler Baker <tyler.baker@linaro.org>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>