HD Split: Avoid redundant upgrades

This avoids repeaded upgrades when support for more multiple keyman references
is added in the next commit:
https://github.com/bitcoin/bitcoin/pull/16341#discussion_r322370108

src/wallet/scriptpubkeyman.cpp

@@ -384,7 +384,7 @@ bool LegacyScriptPubKeyMan::Upgrade(int prev_version, std::string& error)
         hd_upgrade = true;
     }
     // Upgrade to HD chain split if necessary
-    if (m_storage.CanSupportFeature(FEATURE_HD_SPLIT)) {
+    if (m_storage.CanSupportFeature(FEATURE_HD_SPLIT) && CHDChain::VERSION_HD_CHAIN_SPLIT) {
         WalletLogPrintf("Upgrading wallet to use HD chain split\n");
         m_storage.SetMinVersion(FEATURE_PRE_SPLIT_KEYPOOL);
         split_upgrade = FEATURE_HD_SPLIT > prev_version;