Use correct number of channels for the selected CoreAudio stream

The kAudioStreamPropertyPhysicalFormat query returns the number
of channels for the device's first audio stream. If any single,
subsequent stream is selected to fulfill the user request, it may
use a different number of channels. This mismatch can cause a lot
of problems.

RtAudio.cpp

@@ -1782,7 +1782,7 @@ bool RtApiCore :: probeDeviceOpen( unsigned int deviceId, StreamMode mode, unsig
   stream_.deviceFormat[mode] = RTAUDIO_FLOAT32;
 
   if ( streamCount == 1 )
-    stream_.nDeviceChannels[mode] = description.mChannelsPerFrame;
+    stream_.nDeviceChannels[mode] = streamChannels;
   else // multiple streams
     stream_.nDeviceChannels[mode] = channels;
   stream_.nUserChannels[mode] = channels;