clk: fixed_rate: add dummy disable() function

commit 6bf6d81c ("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>