clk: renesas: div6: Use clamp() instead of clamp_t()

As "div" is already "unsigned int", adding "U" suffixes to the constants
"1" and "64" allows us to replace the call to clamp_t() by a call to
clamp().  This removes hidden casts, and thus helps the compiler doing a
better job at type-checking.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/2670c1e3c82a245666578cbbd1fb20d37932fd8e.1617281699.git.geert+renesas@glider.be