clk: shmobile: div6: Avoid division by zero in .round_rate()

Anyone may call clk_round_rate() with a zero rate value, so we have to
protect against that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>