clocksource/drivers/sh_cmt: Remove unneeded memset() in sh_cmt_setup()

Memory for cmt struct is allocated by kzalloc() in sh_cmt_setup.

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