their programmer doesn't provide a (low-level) cmd_tpi method;
instead, fall back to the normal programmer methods which are
supposed to handle the situation.
This fixes a regression where the recent bitbang-TPI implementation
broke TPI handling of STK600/AVRISPmkII.
git-svn-id: svn://svn.savannah.nongnu.org/avrdude/trunk@1008 81a1dc3b-b13d-400b-aceb-764788c761c2