clk: atlas7: fix integer overflow in dto rate calculation

I cannot believe that I spend quite a lot time in finding this bug.
It seems a pitfall people tend to fall in.

In "int64 = int32 * int32", conversion from 32-bits to 64-bits comes
after the multiplication. So this statement may not work as expected.

Signed-off-by: Yibo Cai <yibo.cai@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>