clk: fixed_rate: add dummy disable() function

commit 6bf6d81c1112 ("clk: fixed_rate: add dummy enable() function")
implemented .enable, so fixed rate clocks can be used where drivers
might call clk_enable(). Implement the .disable op for the same reason;
some drivers, e.g. USB PHYs, may attempt to disable clocks at runtime.

Signed-off-by: Samuel Holland <samuel@sholland.org>
diff --git a/drivers/clk/clk_fixed_rate.c b/drivers/clk/clk_fixed_rate.c
index e0dc4ab..c5a2a42 100644
--- a/drivers/clk/clk_fixed_rate.c
+++ b/drivers/clk/clk_fixed_rate.c
@@ -26,6 +26,7 @@
 const struct clk_ops clk_fixed_rate_ops = {
 	.get_rate = clk_fixed_rate_get_rate,
 	.enable = dummy_enable,
+	.disable = dummy_enable,
 };
 
 void clk_fixed_rate_ofdata_to_plat_(struct udevice *dev,