Reduce the "rain forest" of pointers a bit.

- Use the channel info from the drive instead of from hwgroup where possible.
   Since we have now a direct association between a channel and irq, we can fix
   the handler for unexpected interrupts to don't have to iterate over multiple
   possibilities.

- Finally it was just possible to remove the circular list of channels
   associated with a hwif. The hwif member of struct hwgroup_s as well as
   next member if struct ata_channel are gone.

- Unify ide_build_sglist() and raw_build_sglist() in to one function.  Use the
   queue associated with a request in the case of a CMD request there instead of
   referencing the device minor number. This will be pushed even further later.