In xhci_check_transfer, we tried to iterate through STREAM * NUM_SLOTS
and find the correct stream. However, stream is rear nowadaya both on
FreeBSD and Linux. In which means that such STREAM * NUM_SLOTS array is
sparse.
As a result, we use a bit map to map stream id to slot. For a
xhci_mask[stream_id], it contains the slot_id bitmap over which slot has
been occupied.