[PATCH] libata: use ata_id_xfermask() in ata_dev_configure()

Replace quick & dirty max transfer mode determination in
ata_dev_configure() with ata_id_xfermask().  While at it, rename
xfer_modes variable to xfer_mask and make it unsigned int for
consistency.

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