powercap/dtpm_cpu: Reset per_cpu variable in the release function

The release function does not reset the per cpu variable when it is
called. That will prevent creation again as the variable will be
already from the previous creation.

Fix it by resetting them.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220130210210.549877-2-daniel.lezcano@linaro.org