Comment by pebers

Interesting, did not know that. He implies (but doesn't say flat-out and it could indeed be a retcon) in the foreword to The Hobbit that it's deliberate:

  In English the only correct plural of dwarf is dwarfs, and the adjective
  is dwarfish. In this story dwarves and dwarvish are used, but only when
  speaking of the ancient people to whom Thorin Oakenshield and his 
  companions belonged.

I've not heard of any usage of it pre-Tolkien though, and can't find anything obvious after a bit of searching (mostly "dwarvish" turns up Tolkien related things). Anything you can point me to there?