Simplify and improve gw16012_handle_parport_command:

- Show the port number to the user when asking for it or setting it.
- Print an error if the parport_port has already been set.
- Use parse_u16 helper to ensure the parport_port string parses correctly.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2211 b42882b7-edfa-0310-969c-e2dbd0fdcd60