Make this code look more like the rest of the OpenOCD code.

 - Use calloc() directly, not NEW() ... and fix some potential
   memory leaks while we're at it.

 - Remove FNC_INFO ... it's a NOP that just clutters things,
   and it's trivial for developers to add tracing as needed.

 - Replace FNC_INFO_NOTIMPLEMENTED with LOG_WARNING calls;
   ditto.  And stop having those call sites wrongly succeed!

 - Waste less space with the CHECK_RETVAL() macro.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>