Make sure DMA buffer is zeroed in 32 bits mode. (AUD-594)

[hallgrim]

DMA buffers for I2S must be set to zero to prevent noise after playback stops. This works well in 16 bits mode. In 32 bits mode leaves some samples in the buffer and these samples repeat.

The existing code created a buffer of four times the number of samples in the DMA buffer. I think this is a hardcoded assumption of 16bits stereo (4 bytes per sample).
i2s->config.i2s_config.dma_buf_len * 4

The number of bytes in the DMA buffer cannot be reliably calculated. The logic is non-trivial (max 4096, etc.). It turns out that the I2S library already have a function to zero the buffer i2s_zero_dma_buffer. When I call i2s_zero_dma_buffer(i2s->config.i2s_port); instead of i2s_write(i2s->config.i2s_port, (char *)buf, i2s->config.i2s_config.dma_buf_len * 4, &bytes_written, portMAX_DELAY); I get no noise, neither in 16 or 32 bits mode.

There is a test on i2s->config.i2s_config.mode & I2S_MODE_DAC_BUILT_IN that sets the buffer to 0x80 instead of 0x00. I left this branch as it was with i2s_write instead of i2s_zero_dma_buffer, but I have not been able to test that there are no regression with the I2S_MODE_DAC_BUILT_IN.

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[Alvin1Zhang]

@hallgrim Thanks for the contribution.