staging: unisys: correctly NULL-terminate visorbus sysfs attribute array

I'm not sure what adverse runtime effects the previously-omitted
NULL-termination would cause, but the code was definitely wrong.

Fixes: 79573162 ('staging: unisys: Clean up device sysfs attributes')
Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>