Add ata_pack_xfermask(), ata_xfer_mask2mode(), ata_xfer_mode2mask(),
ata_xfer_mode2shift() and ata_id_xfermask().  These functions will be
used by following patches to simplify xfer_mask handling.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>