You really can't argue in favor of doughnut based on grammar, although I certainly have a temptation to because donuts are made out of dough. It seems obvious except what is the nut, then? They're so good they make you nut? They usually don't have nuts, even hipster places don't top them with nuts most of the time.

The nut could be the hole, but... thassa donut hole. So doughnut makes no sense at the end of the day.

Just spell it phonetically- donut.