[PATCH] update dtc2278 driver

[PATCH] update dtc2278 driver

- common dtc2278_init() for built-in and module
- touch hwifs only if hwif->chipset == ide_unknown for both ports,
  so we don't thrash already used hwifs when loading module
- release hwif only if hwif->chipset == ide_dtc2278
- mark exit functions with __exit
- do not use ide_hwifs[] directly
- minor cleanups