clk: renesas: rcar-gen3: Improve arithmetic divisions

- Use div64_ul() instead of div_u64() if the divisor is unsigned long,
    to avoid truncation to 32-bit on 64-bit platforms,
  - Use div_u64() for 64-by-32 divisions.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/20190830134515.11925-3-geert+renesas@glider.be

drivers/clk/renesas/rcar-gen3-cpg.c

@@ -122,10 +122,10 @@ static long cpg_z_clk_round_rate(struct clk_hw *hw, unsigned long rate,
 	unsigned int mult;
 
 	prate = *parent_rate / zclk->fixed_div;
-	mult = div_u64(rate * 32ULL, prate);
+	mult = div64_ul(rate * 32ULL, prate);
 	mult = clamp(mult, 1U, 32U);
 
-	return (u64)prate * mult / 32;
+	return div_u64((u64)prate * mult, 32);
 }
 
 static int cpg_z_clk_set_rate(struct clk_hw *hw, unsigned long rate,