nrf: i2s: tune audio buffering
nrf: i2s: tune audio buffering

.. based on some tasks I found that caused stuttering:

 # Test SD and printing
 while True: os.listdir('.')

 # Test bulk I/O
 while True: len(open('somefile.wav', 'rb').read())

Each of these tasks *WAS* worse and I am improving them in a separate
PR by adding RUN_BACKGROUND_TASKS to them.

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