msm: clock: Return early if same rate used in clk_set_rate()

Now that the rate field is cached inside struct clk we can move
the check for 'rate == current rate' to the toplevel clock.c
code. This way each hardware specific driver doesn't have to
duplicate code and check for this condition in their
clk_set_rate() ops.

Change-Id: I5350ec61683713f7f9dd5c278b44a7c27463df1a
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>