Fix gain_db renamed _gain

Author: pennam

libraries/PDM/src/stm32/PDM.cpp

@@ -80,7 +80,7 @@ int PDMClass::begin(int channels, int sampleRate) {
   g_pcmbuf = (uint16_t*)_doubleBuffer.data();
   _doubleBuffer.swap(0);
 
-  if(py_audio_init(channels, sampleRate, gain_db, 0.9883f)) {
+  if(py_audio_init(channels, sampleRate, _gain, 0.9883f)) {
     py_audio_start_streaming();
     _init = 1;
     return 1;