Be more careful when configuring MCLK

output_i2s.cpp

 
 	// enable MCLK output
 	I2S0_MCR = I2S_MCR_MICS(MCLK_SRC) | I2S_MCR_MOE;
+	while (I2S0_MCR & I2S_MCR_DUF) ;
 	I2S0_MDR = I2S_MDR_FRACT((MCLK_MULT-1)) | I2S_MDR_DIVIDE((MCLK_DIV-1));
 
 	// configure transmitter

output_i2s_quad.cpp

 
 	// enable MCLK output
 	I2S0_MCR = I2S_MCR_MICS(MCLK_SRC) | I2S_MCR_MOE;
+	while (I2S0_MCR & I2S_MCR_DUF) ;
 	I2S0_MDR = I2S_MDR_FRACT((MCLK_MULT-1)) | I2S_MDR_DIVIDE((MCLK_DIV-1));
 
 	// configure transmitter

output_spdif.cpp

 
 	// enable MCLK output
 	I2S0_MCR = I2S_MCR_MICS(MCLK_SRC) | I2S_MCR_MOE;
+	while (I2S0_MCR & I2S_MCR_DUF) ;
 	I2S0_MDR = I2S_MDR_FRACT((MCLK_MULT-1)) | I2S_MDR_DIVIDE((MCLK_DIV-1));
 
 	// configure transmitter