staging: unisys: make visorbus_dev_groups static

visorbus_dev_groups is not referenced outside visorbus_main.c, so it can
be declared static.

Found using sparse.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>