adi_jtag_dp_scan_u32() now wraps adi_jtag_dp_scan(), removing
code duplication. Include doxygen for the former. Comment
some particularly relevant points. Minor fault handling fixes
for both routines: don't register a callback that can't run,
or return ERROR_OK after an error.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>