clk: rockchip: drop old_rate calculation on pll rate changes

Previously when everything happened in the set_rate callbacks itself we
needed the old_rate value for the possible rate rollback, so that made
it easy to also use it in the debug output.

Now with the param-handling being done in separate functions, reading and
recalculating the current pll rate only to use it in a debug message that
won't get displayed in regular cases anyway is quite a waste.

Therefore drop that value from the debug output. In the worst case that
previous rate will have been displayed on the rate change before.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>