Add some battle frontier constant usage

src/party_menu.c

@@ -5647,11 +5647,11 @@ static u8 GetBattleEntryLevelCap(void)
     case FACILITY_MULTI_OR_EREADER:
         return MAX_LEVEL;
     case FACILITY_UNION_ROOM:
-        return 30;
+        return UNION_ROOM_MAX_LEVEL;
     default: // Battle Frontier
         if (gSpecialVar_0x8004 == FRONTIER_LVL_50)
-            return 50;
-        return MAX_LEVEL;
+            return FRONTIER_MAX_LEVEL_50;
+        return FRONTIER_MAX_LEVEL_OPEN;
     }
 }