[AGPGART] Mark maxes_table as const

It's never written to.

Noted by Arjan van de Ven
Signed-off-by: Dave Jones <davej@redhat.com>

drivers/char/agp/backend.c

@@ -97,7 +97,7 @@ void agp_backend_release(struct agp_bridge_data *bridge)
 EXPORT_SYMBOL(agp_backend_release);
 
 
-static struct { int mem, agp; } maxes_table[] = {
+static const struct { int mem, agp; } maxes_table[] = {
 	{0, 0},
 	{32, 4},
 	{64, 28},