clk: renesas: Use str_on_off() helper

Use the str_on_off() helper instead of open-coding the same operation.
Note that this does change the case of the flags, which doesn't matter
much for debug messages.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/622f8554dcb815c8fc73511a1a118c1724570fa9.1745840497.git.geert+renesas@glider.be