clk: renesas: rcar-gen3: Absorb cpg_sd_clock_calc_div()

cpg_sd_clock_round_rate() is the sole caller of cpg_sd_clock_calc_div(),
hence absorb the latter into the former.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20190830134515.11925-5-geert+renesas@glider.be