staging: vboxvideo: Fix line size exceeding 80 columns.

This patch fixes the checkpatch.pl warning:

WARNING: line over 80 characters
+               vbva = (void __force *)vbox->vbva_buffers + i *
VBVA_MIN_BUFFER_SIZE;

Signed-off-by: Daniel Junho <djunho@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>