[PATCH] libata: use xfer_mask helpers in ata_dev_set_mode()

Rewrite hardcoded xfer_mode string determination in ata_dev_set_mode()
using xfer_mask helpers.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>