[dts] [PATCH V1] tests/TestSuite_shutdown_api:fixed differences brought about by speed units

[Tu, Lijuan]

> > Can you write clearly why do we need to do this change ?
> 
> if change Link speed is1000 or 10000,the DUT's Link speed is 1 Gbps or 10 Gbps,
> if change Link speed is100, the DUT's Link speed is 100 Mbps

It's not a good idea to get unit by value, if the speed is 1000Gbps(although we don't have it now, but soon in future), it can't know it is Gbps or Mbps.
Generally speaking, get unit first and then verify value.