This commits adds a new error code that can be return from a
registered control callback: USBD_REQ_NEXT_CALLBACK. This return code
signifies that the callback is done processing the data successfully,
but user would like to have all matching callbacks down the callback
chain to be executed too.
This change allows for example to intercept standard requests like
GET_DESCRIPTOR, do some small action upon receiving of one, but still
have the standard callback executed and do it's job. This way user
doesn't have to re-implement standard GET_DESCRIPTOR functionality if
they want to intercept that request to do some small thing.
c5c4db0196