clocksource/drivers/h8300_timer8: Retrieve the clock rate at init time

The current code retrieves the rate value when the timer is enabled which
occurs each time a timer is re-armed. Except if the clock frequency has changed
magically I don't see why this should be done each time.

Retrieve the clock rate value at init time only.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>