clk: skip unnecessary set_phase if nothing to do

Let's compare the degrees from clk_set_rate with
clk->core->phase. If the requested degrees is already
there, skip the following steps.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
[sboyd@codeaurora.org: s/drgrees/degrees/ in commit text]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>