Factoring of jtag_examine_chain for maintainability:

- Improve variable type: change device_count to unsigned.
- Improves jtag_tap_count_enabled() API too (now returns unsigned).


git-svn-id: svn://svn.berlios.de/openocd/trunk@2091 b42882b7-edfa-0310-969c-e2dbd0fdcd60