- Push BAD_DMA_DRIVE and GOOD_DMA_DRIVE to the ide-pmac.c file, since this is
   the only place where those get used.

- Kill unused fields from the ide_task_s structure. In esp. we pass a task
   attached to a request and not the other way around!

- Rename hwif field to channel in struct ide_drive_s.

- Move the request queue to the level where proper serialization has to happen
   anyway - the channel structure.