usb: Refactor endpoint 0 IN packet handling, phase 1
usb: Refactor endpoint 0 IN packet handling, phase 1

Phase 1 is to copy and paste the code into a new function, without fixing
the indenting.  This way this diff looks clean.  Unlike the SETUP and OUT
refactoring, this diff came out straight-forward.

pololu/m-stack,rollingstone/m-stack,rollingstone/m-stack,pololu/m-stack