QTBUG-18435 fix stored procedure output parameters on ODBC

QTBUG-18435 fix stored procedure output parameters on ODBC

Follow-up to c55a99965d8c08d5f924d49db4fe4aa49df8096.

3 problems prevented stored procedure output parameters from working.
- SQLBindParameter needs access to buffer provided by QByteArray.
- The length of the buffer is measured in bytes.
- A typo corrupted conversion back to QString.

Also, data() makes more sense than constData() to expose the buffer.

Task-Id: QTBUG-18435
Change-Id: I66444b13c0f584ed79bcf026e5a23caff83c22cb
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Mark Brand <mabrand@mabrand.nl>