Make CPURead/WriteFunc structure 'const'

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/hw/onenand.c b/hw/onenand.c
index 84fdb21..59bc8ac 100644
--- a/hw/onenand.c
+++ b/hw/onenand.c
@@ -603,13 +603,13 @@
     }
 }
 
-static CPUReadMemoryFunc *onenand_readfn[] = {
+static CPUReadMemoryFunc * const onenand_readfn[] = {
     onenand_read,	/* TODO */
     onenand_read,
     onenand_read,
 };
 
-static CPUWriteMemoryFunc *onenand_writefn[] = {
+static CPUWriteMemoryFunc * const onenand_writefn[] = {
     onenand_write,	/* TODO */
     onenand_write,
     onenand_write,