staging/vboxvideo: don't set dev_priv_size = 0

The compiler already clears this for us.

More important, someone might look what this is actually used for,
and freak out about the dragon staring back at them.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Nicholas Mc Guire <der.herr@hofr.at>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Fabio Rafael da Rosa <fdr@pid42.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20190204103114.30772-1-daniel.vetter@ffwll.ch