clk: rockchip: validity should be checked prior to cpu clock rate change

If validity is not checked prior to clock rate change, clk_set_rate(
cpu_clk, unsupported_rate) will return success, but the real clock rate
change operation is prohibited in post clock change event. Alough post
clock change event will report error due to unsupported clock rate is
set, but this error message is ignored by clock framework.

Signed-off-by: Elaine Zhang <zhangqing@rock-chips.com>
Tested-by: Rocky Hao <rocky.hao@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>