Simplify and fix bug in jtag_tap_by_string:

- Bug fix: Use unsigned type and strtoul when parsing for position number.
- Simplify logic by returning directly when a tap is found by name.
- Reduce scope: declare temporary variables with first use.
- Bring code up to current style guidelines.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2141 b42882b7-edfa-0310-969c-e2dbd0fdcd60