i2c: riic: avoid potential division by zero

[ Upstream commit 7890fce6 ]

Value comes from DT, so it could be 0. Unlikely, but could be.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>