cpufreq: Return 0 from ->fast_switch() on errors

CPUFREQ_ENTRY_INVALID is a special symbol which is used to specify that
an entry in the cpufreq table is invalid. But using it outside of the
scope of the cpufreq table looks a bit incorrect.

We can represent an invalid frequency by writing it as 0 instead if we
need. Note that it is already done that way for the return value of the
->get() callback.

Lets do the same for ->fast_switch() and not use CPUFREQ_ENTRY_INVALID
outside of the scope of cpufreq table.

Also update the comment over cpufreq_driver_fast_switch() to clearly
mention what this returns.

None of the drivers return CPUFREQ_ENTRY_INVALID as of now from
->fast_switch() callback and so we don't need to update any of those.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>