Prevent calling USB backends multiple times.

Prevent calling USB backends multiple times.

The TRB processing loop could potentially call a back-end twice
with the same status transaction. While this was generally benign,
some code paths in the tablet backend weren't set up to handle
this case, resulting in a NULL dereference.

Fix by
 - returning a STALL error when an invalid request was seen in the backend
 - skipping a call to the backend if the number of packets in a status
   transaction was zero (this code fragment was taken from the Intel ACRN
   xhci backend)

PR:	246964
Reported by:  Ali Abdallah
Discussed with: Leon Dang (author)
Reviewed by: jhb (#bhyve), Leon Dang
Approved by: jhb
Obtained from:  Intel ACRN (partially)
MFC after: 2 weeks
Differential Revision:	https://reviews.freebsd.org/D25228