tile: array underflow in setup_maxnodemem()

My static checker correctly complains that we should have a lower bound
on "node" to prevent an array underflow.

Fixes: 867e359b

 ("arch/tile: core support for Tilera 32-bit chips.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>