of_get_required_opp_performance_state() returns 0 on errors currently
and a positive performance state otherwise. Since 0 is a valid
performance state (representing off), it would be better if this routine
returns negative values on error.

That will also make it behave similar to
dev_pm_opp_xlate_performance_state(), which also returns performance
states and returns negative values on error. Change the return type of
the function to "int" in order to return negative values.

This doesn't have any users for now and so no other part of the kernel
will be impacted with this change.

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>