Improve VID/PID command argument handling in FTDI driver:

- Bug fix: Return a syntax error when less than two arguments are given.
- Bug fix: Use parse_u16 helper to ensure vales are parsed properly.
- Simplify loop termination logic by ensuring argc is always even.
- Move loop induction variable declaration to where it is used.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2215 b42882b7-edfa-0310-969c-e2dbd0fdcd60