pata_it821x: (partially) fix DMA in RAID mode

Code intended to check DMA status was checking DMA command register.

Moreover firmware seems to "forget" to set DMA capable bit for the
slave device (at least in RAID mode but without ITE RAID volumes) so
check device ID for DMA capable bit when deciding whether to use DMA
and remove DMA status check completely.

Thanks to Pavol Simo for the bugreport and testing the initial fix.

This change unfortunately still doesn't fix DMA in RAID mode (which
works fine with IDE it821x) but Alan is working on the missing pieces
(pata_it821x vs libata EH issues).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>