directsoundsink: simplify how DirecSoundBuffer is cleared

we always want to clear the whole buffer so no need to
start from offset even if the offset is always zero.

https://bugzilla.gnome.org/show_bug.cgi?id=788847

sys/directsound/gstdirectsoundsink.c

@@ -815,7 +815,7 @@ gst_directsound_sink_reset (GstAudioSink * asink)
 
     /*reset the buffer */
     hRes = IDirectSoundBuffer_Lock (dsoundsink->pDSBSecondary,
-        dsoundsink->current_circular_offset, dsoundsink->buffer_size,
+        0, dsoundsink->buffer_size,
         &pLockedBuffer, &dwSizeBuffer, NULL, NULL, 0L);
 
     if (SUCCEEDED (hRes)) {