i2c: rcar: Remove superfluous call to clk_get_rate()

Variable "rate" already contains the current clock rate, so use that
rather than calling clk_get_rate() again.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>