ide: clear bmdma status in ide_intr() for ICHx controllers (revised #4)

patch 1/2 (revised):
- Fix drive->waiting_for_dma to work with CDB-intr devices.
- Do the dma status clearing in ide_intr() and add a new
  hwif->ide_dma_clear_irq for Intel ICHx controllers.

Revised per Alan, Sergei and Bart's advice.

Patch against 2.6.20-rc6. Tested ok on my ICH4 and pdc20275 adapters.
Please review/apply, thanks.

Signed-off-by: Albert Lee <albertcc@tw.ibm.com>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: "Adam W. Hawks" <awhawks@us.ibm.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>