clocksource/drivers/renesas-ostm: Use DIV_ROUND_CLOSEST() helper

Use the DIV_ROUND_CLOSEST() helper instead of open-coding the same
operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>