Staging: unisys: virthba: Fix variable length array

A character array was declared on the stack with variable length.  This has
been corrected to use a fixed length.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ken Cox <jkc@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>