qsql_oci: fix option types

These are signed values. Notice the initialization to -1, which
apparently never worked. The unsigned types previously used may
be due to confusion with other arguments of OCIAttrSet():

sword OCIAttrSet ( dvoid       *trgthndlp,
                   ub4         trghndltyp,
                   dvoid       *attributep,
                   ub4         size,
                   ub4         attrtype,
                   OCIError    *errhp );

Examples found in web searches also use signed int.

Change-Id: I8db273a554fa0ef454a8dfce5d83983f79cf6cb9
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>

src/sql/drivers/oci/qsql_oci.cpp

@@ -497,8 +497,8 @@ public:
     OCIError *err;
     bool transaction;
     int serverVersion;
-    ub4 prefetchRows;
-    ub2 prefetchMem;
+    int prefetchRows;
+    int prefetchMem;
     QString user;
 
     void allocErrorHandle();