ESP32 Sound output stopped working with IDF >= 4.2.0

[judge2005]

There were quit a few changes around the use of mclk in the Espressif IDE. My hardware doesn't use that signal and to get the code working again I had to call SetMclk(false) on the instance of AudioOutputI2S. There are some comments in the source code to the effect that 'mclk' isn't used, but this isn't the case if use_mclk is true. I don't know what the default for that flag should be, except maybe to be backwards-compatible it should default to false.

[lyusupov]

I can confirm that PR #552 broke ESP32 EXT no-mclk I2S function in past and this PR gives a positive fix.