Name
usb_free_urb — 
     frees the memory used by a urb when all users of it are finished
 
Arguments
- urb
- 
     pointer to the urb to free, may be NULL
     
Description
   Must be called when a user of a urb is finished with it.  When the last user
   of the urb calls this function, the memory of the urb is freed.
Note
   The transfer buffer associated with the urb is not freed unless the
   URB_FREE_BUFFER transfer flag is set.