i2c: rcar: calculate divider instead of brute-forcing it

Instead of trying all values, we can actually compute it as the comment
suggests. It is unclear what the comment means with "involved", it works
nicely.

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>