Simplify jtag_add_plain_dr_scan:

- Use jtag_set_error instead of accessing jtag_error directly.
- Wrap function arguments to fit everything in 80 columns.
- Move retval variable to location of first use.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2121 b42882b7-edfa-0310-969c-e2dbd0fdcd60