Simplify jtag_add_sleep:

- Add todo for removing keep_alive: is this a layering violation?
- Use jtag_set_error instead of accessing jtag_error directly.
- Remove superfluous retval temporary variable and empty return.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2127 b42882b7-edfa-0310-969c-e2dbd0fdcd60