ide: switch set_xfer_rate() to use REQ_TYPE_ATA_TASKFILE requests

Based on the earlier work by Tejun Heo.

Switch set_xfer_rate() to use REQ_TYPE_ATA_TASKFILE requests
and make ide_wait_cmd() static.

There should be no functionality changes caused by this patch.

Cc: Tejun Heo <htejun@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>