sound/soc: dwc-i2s: choose FIFO thresholds based on DMA burst constraints

Valid ranges for the I2S peripheral's FIFO configuration include a depth
of 16 - unconditionally setting the burst length to 16 with a fifo
threshold of size/2 will cause under/overflows.

For DMA engines with restricted capabilities the requested burst length
and FIFO thresholds need to be adjusted downward accordingly.

Both the RX and TX FIFOs operate on "less-than" thresholds. Setting the
TX threshold to fifo_size minus burst means the FIFO is kept nearly-full.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>