nouveau/nvkm/subdev/clk/gk20a.c: fix wrong do_div() usage

do_div() must only be used with a u64 dividend.
Signed-off-by: Nicolas Pitre <nico@linaro.org>

drivers/gpu/drm/nouveau/nvkm/subdev/clk/gk20a.c

@@ -141,9 +141,8 @@ gk20a_pllg_calc_rate(struct gk20a_clk *clk)
 
 	rate = clk->parent_rate * clk->n;
 	divider = clk->m * pl_to_div[clk->pl];
-	do_div(rate, divider);
 
-	return rate / 2;
+	return rate / divider / 2;
 }
 
 static int