INT 78 - TARGA.DEV - SET/CLEAR EARLY RETURN MODE
        AH = 06h
        AL = 00h clear early return mode
           = 01h set early return mode
Note:   if early return mode is set then SCSI will return with no errors
          when the last DMA transfer is started in a call with AH=13h or AH=14h
        if this routine is not called, early return mode is cleared
SeeAlso: AH=14h,AH=15h