firmware: handle zero-length-packets manually for the out control endpoint

[Qyriad]

With greatscottgadgets/libgreat#15 and greatscottgadgets/facedancer#22, fixes #283.

[ktemkin]

The commit message here should indicate that this is

1. for GreatDancer (ZLTs are still automatically generated for normal-USB EP0)
2. the commit message should probably be shorter

[ktemkin]

The commit could probably use an explanation in the commit message, too. Something explain why Linux is silly would be good. ;)

Another question to be answered is if we should have a paralleling change in the normal firmware. I suspect we suffer exactly the same issue (we don't accept packets from OS's cough Linux cough that don't properly generate ZLPs at the end of their OUT control requests).