cpufreq: Make cpufreq_driver->exit() return void

The cpufreq core doesn't check the return type of the exit() callback
and there is not much the core can do on failures at that point. Just
drop the returned value and make it return void.

Signed-off-by: Lizhe <sensor1010@163.com>
[ Viresh: Reworked the patches to fix all missing changes together. ]
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> # Mediatek
Acked-by: Sudeep Holla <sudeep.holla@arm.com> # scpi, scmi, vexpress
Acked-by: Mario Limonciello <mario.limonciello@amd.com> # amd
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com> # bmips
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Acked-by: Kevin Hilman <khilman@baylibre.com> # omap