Staging: unisys: visorbus: Make a read-only structure

This patch add const qualifier at the declaration of the structure.
The structure become a read-only data, this increase the security.

Signed-off-by: Mihaela Muraru <mihaela.muraru21@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>