exosphere: Fix virtual mapping of MC_SECURITY_CFG3.

Allow DRAM magic test value to be written on < 4.0.0.

exosphere/src/warmboot_main.c

@@ -54,7 +54,7 @@ void __attribute__((noreturn)) warmboot_main(void) {
     identity_unmap_iram_cd_tzram();
 
     /* On warmboot (not cpu_on) only */
-    if (MC_SECURITY_CFG3 == 0) {
+    if (VIRT_MC_SECURITY_CFG3 == 0) {
         if (!configitem_is_retail()) {
             /* TODO: uart_log("OHAYO"); */
         }