Fix for Soapy (timing issue for SetupStream)

[OhSoGood]

Hi,
On my PC with test_xtrx_soapy I encountered a weird bug preventing exactly once every two time test_xtrx_soapy to run, more precisely it was freezing when reading the stream. I manage to circumvent it (so that's rather a hack than a fix actually) by adding usleep(20000) after SoapySDRDevice_setupStream but before SoapySDRDevice_activateStream.

As the issue is time-based, it may be linked with the PC speed so, for info, my PC is a Ryzen 2700 running Ubuntu 18.04