usbd: clear the queued-setup counter on bus reset

A SETUP counted before a bus reset must not be carried across it: the
consumer would either skip a post-reset SETUP (count drained by the
stale entry) or, if the count leaked high for any other reason, skip
them all. usbd_reset() now zeroes the counter; the consumer already
guards on zero, and any pre-reset SETUP still in the queue is stale by
definition and correctly discarded.