clk: renesas: rcar-gen3-cpg: Refactor checks for accessing the div table

Do the checks for accessing the SD divider table only when the rate gets
updated, namely on init and set_rate. In all other cases, reuse the last
value. This simplifies code, runtime load, and error reporting.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>