Set mono or stereo mode with I2S_SLOT_MODE_MONO or I2S_SLOT_MODE_STEREO In TX direction, mono means the written buffer contains only one slot data and stereo means the written buffer contains both left and right data.
i2s_std_slot_config_tslot_cfg;/*!< Standard mode slot configuration, can be generated by macros I2S_STD_[mode]_SLOT_DEFAULT_CONFIG, [mode] can be replaced with PHILIPS/MSB/PCM */