A way to block on I2S driver draining its queue

[ndreys]

I'm using I2S subsystem and I have the following conundrum. I'm calling i2s_trigger(i2s, I2S_DIR_TX, I2S_TRIGGER_DRAIN) to finish playing a waveform which puts the IP block driver into I2S_STATE_STOPPING state. But since that "trigger" call is non-blocking, it's very easy for me to end up trying to perform other I2S calls which fail unless the driver moves past the "stopping" state. There doesn't seem to be any API to block until the driver is done draining. Is that right or did I miss something? My current workaround for this is to have a loop that allocates all of the buffers in the I2S SLAB and then immediately frees them, which is kind of silly in how heavy handed it is. If this is API miss, what should the new API should be like?

cc: @anangl @TomasBarakNXP