input/alsa: rebase on AsyncInputStream

Use the snd_pcm_t only in the IOThread, and reuse code that is
well-known to work.

src/input/AsyncInputStream.cxx

@@ -215,6 +215,17 @@ AsyncInputStream::Read(void *ptr, size_t read_size, Error &error)
 	return nbytes;
 }
 
+void
+AsyncInputStream::CommitWriteBuffer(size_t nbytes)
+{
+	buffer.Append(nbytes);
+
+	if (!IsReady())
+		SetReady();
+	else
+		cond.broadcast();
+}
+
 void
 AsyncInputStream::AppendToBuffer(const void *data, size_t append_size)
 {