directsoundsink: simplify how DirecSoundBuffer is cleared
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

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good