Add comments about I2S FIFO reset bits (was an issue on I2S test fixture)

input_i2s.cpp

 	//delayMicroseconds(500);
 	//digitalWriteFast(3, LOW);
 
+	// TODO: should we set & clear the I2S_RCSR_SR bit here?
 	AudioOutputI2S::config_i2s();
 
 	CORE_PIN13_CONFIG = PORT_PCR_MUX(4); // pin 13, PTC5, I2S0_RXD0

output_i2s.cpp

 	block_left_1st = NULL;
 	block_right_1st = NULL;
 
+	// TODO: should we set & clear the I2S_TCSR_SR bit here?
 	config_i2s();
 	CORE_PIN22_CONFIG = PORT_PCR_MUX(6); // pin 22, PTC1, I2S0_TXD0