Fix from blargg-adb-usbasp-3.zip

- http://geekhack.org/index.php?topic=14290.msg1118206#msg1118206
* Now merges extra ADB events with next where possible, making better
* use of limited USB update opportunities.
* Eliminated adjustable ADB polling rate, as higher rates only hurt, and
* improved USB synchronization made adjustable polling impossible.
* Consolidated sources.
* Using 16-bit timer1 for everything.