drivers: i2s: Fix TX transfer stopping issue in some corner cases

In previous implementions, when there is no buffers in TX queue in a TX
callback context, TX will be stopped immediately. It is not correct
because there may still TX blocks in DMA TX transfer queue.
TX should only be stopped when DMA queue is empty and no more in I2S TX
queue.
Another modification is to set correct TX water FIFO level.

Signed-off-by: Raymond Lei <raymond.lei@nxp.com>