Added XUA_I2S_N_BITS (was N_BITS_I2S) and tidied up usage of define

lib_xua/api/xua_conf_default.h

@@ -193,6 +193,16 @@
     #define I2S_DOWNSAMPLE_CHANS_IN I2S_CHANS_ADC
 #endif
 
+/**
+ * @Brief Number of bits per channel for I2S/TDM
+ *
+ * Default: 32 bits
+ */
+#ifndef XUA_I2S_N_BITS
+#define XUA_I2S_N_BITS (32)
+#endif
+
+
 /**
  * @brief Max supported sample frequency for device (Hz). Default: 192000
  */