'outlen' becomes zero after n64_txn function on some AVRs

[GhostSonic21]

I know the last commit was about 2 years ago, but I wanted to at least have this for anyone else like me who found this repo and was having trouble with it.

I tried to run this on my Arduino Mega 2560, an ATmega2560 16Mhz board. Was having where commands were being sent to the Arduino, but no data whatsoever was coming back. I determined that n64_txn does finish, but the 'outlen' variable was somehow zeroing itself.

Setting the outlen variable to 'volatile' seems to fix the issue, just not sure if this is the proper way to fix it as I'm not terribly familiar with inline asm for AVRs.