Name
spi_busnum_to_master — 
     look up master associated with bus_num
 
Arguments
- bus_num
- 
     the master's bus number
     
Description
   This call may be used with devices that are registered after
   arch init time.  It returns a refcounted pointer to the relevant
   spi_master (which the caller must release), or NULL if there is
   no such master registered.
Return
   the SPI master structure on success, else NULL.