nmcli: connection: don't overwrite port-type if explicitly set

When processing the "type" property we deduce the port-type in some
cases and set it. If the user has chosen a port-type we must not
overwrite it. In any case, we should raise an error when validating the
connection.

Fixes: c5324ed285 ('nmcli: streamline connection addition')

src/nmcli/connections.c

@@ -4616,9 +4616,12 @@ set_connection_type(NmCli            *nmc,
                     gboolean          allow_reset,
                     GError          **error)
 {
+    NMSettingConnection *s_con     = nm_connection_get_setting_connection(con);
     GError              *local     = NULL;
     const char          *port_type = NULL;
 
+    nm_assert(s_con);
+
     value = check_valid_name_toplevel(value, &port_type, &local);
     if (!value) {
         if (!allow_reset)
@@ -4632,7 +4635,7 @@ set_connection_type(NmCli            *nmc,
         return FALSE;
     }
 
-    if (port_type) {
+    if (!nm_setting_connection_get_port_type(s_con) && port_type) {
         if (!set_property(nmc->client,
                           con,
                           NM_SETTING_CONNECTION_SETTING_NAME,