selftests: mlxsw: Make the unsplit array global in port_scale test

Currently, the array of the ports that were split in the port_scale test
is local, so the port_cleanup() unsplits an empty array.

Make the array global so the cleanup will be preformed properly.

Suggested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>