NCR53C9x: Add missing slave_{alloc,destroy}() (from Kars de Jong and Matthias
Urlichs). This affects the following drivers:
  - Amiga Blizzard 1230, Blizzard 2060, CyberStorm, CyberStorm Mk II, Fastlane,
    and Oktagon SCSI
  - DECstation NCR53C94 SCSI
  - Jazz ESP 100/100a/200 SCSI
  - Mac 53C9x SCSI
  - MCA NCR 53c9x SCSI
  - Sun-3x SCSI (was already fixed on its own)