[dv/uart] Fix uart timescale issue
[dv/uart] Fix uart timescale issue

The timescale change affects this calculation
  realtime a = 1 * 1ns;
  int i = int'(a); // when timeunit is 1ns, i = 1, when timeunit is 1ps,
                   // i = 1000
  // this is the correct solution
  int i = int'(a/1ns);

Signed-off-by: Weicai Yang <4fa8c7762ee4da95e3dee7dae2c3263ea16637f9@google.com>

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