cpufreq: ACPI: make function acpi_cpufreq_fast_switch() static

The acpi_cpufreq_fast_switch() function is local to the source and
does not need to be in global scope, so make it static.

Cleans up sparse warning:
drivers/cpufreq/acpi-cpufreq.c:468:14: warning: symbol
'acpi_cpufreq_fast_switch' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>