clocksource/drivers/h8300_*: Remove unneeded memset()s

Memory for timer16_priv, timer8_priv and tpu_priv structs is
allocated by devm_kzalloc() in corresponding probe functions
of drivers.
No need to zero it one more time.

Signed-off-by: Alexey Klimov <alexey.klimov@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>