simpleaudio-sndfile: fix minimodem tx_interactive

Un-break tx_interactive: minimodem enables tx_interactive whenever the
stream_name is NULL, so go back to using the stream_name as the sndfile
filename.

src/minimodem.c

@@ -674,7 +674,7 @@ main( int argc, char*argv[] )
 
     if ( filename ) {
 	sa_backend = SA_BACKEND_FILE;
-	sa_backend_device = filename;
+	stream_name = filename;
     }
 
     /*

src/simpleaudio-sndfile.c

@@ -166,7 +166,7 @@ sa_sndfile_open_stream(
 		unsigned int rate, unsigned int channels,
 		char *app_name, char *stream_name )
 {
-    const char *path = backend_device;
+    const char *path = stream_name;
 
     int sf_format;
     switch ( sa->format ) {