Simplify jtag_execute_queue:

- Add static inline jtag_error_clear helper to return and clear jtag_error.
- Use new helper to shrink body of function to two lines.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2117 b42882b7-edfa-0310-969c-e2dbd0fdcd60