usbd_hid_interface.h

The previous version did not work on MCUs that only had USB device mode
(compared to OTG) because of the handling of NAK.  And this previous
handling of NAK had a race condition where a new packet could come in
before USBD_HID_SetNAK was called (since USBD_HID_ReceivePacket clears NAK
as part of its operation).  Furthermore, the double buffering of incoming
reports was not working, only one buffer could be used at a time.

This commit rewrites the HID interface code to have a single incoming
buffer, and only calls USBD_HID_ReceivePacket after the user has read the
incoming report (similar to how the VCP does its flow control).  As such,
USBD_HID_SetNAK and USBD_HID_ClearNAK are no longer needed.

API functionality from the user's point of view should be unchanged with
this commit.