Minor optimization use QString midRef instead of mid.

It doesn't create a temporary string.

Change-Id: If5c540422f69e573107d1f5a886ac7174ae99853
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/sql/drivers/sqlite/qsql_sqlite.cpp

@@ -609,7 +609,7 @@ bool QSQLiteDriver::open(const QString & db, const QString &, const QString &, c
     foreach (const QString &option, opts) {
         if (option.startsWith(QLatin1String("QSQLITE_BUSY_TIMEOUT="))) {
             bool ok;
-            const int nt = option.mid(21).toInt(&ok);
+            const int nt = option.midRef(21).toInt(&ok);
             if (ok)
                 timeOut = nt;
         } else if (option == QLatin1String("QSQLITE_OPEN_READONLY")) {