Some ports need an extra operation to ensure that the main task is
awoken so that a queued background task will execute during an ongoing
light sleep.
This removes the need to enable supervisor ticks while I2SOut is operating.
Closes: #3952
.. otherwise, the background callback to load the I2S fifos does not get
run. (I'm not sure this is _correct_ behavior of sleep + background
tasks, but it is the current behavior)
