[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>
diff --git a/drivers/char/agp/backend.c b/drivers/char/agp/backend.c
index 27bca34..80ee17a 100644
--- a/drivers/char/agp/backend.c
+++ b/drivers/char/agp/backend.c
@@ -97,7 +97,7 @@
 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},