[PATCH] libata: improve xfer mask constants and update ata_mode_string()

Add ATA_BITS_*, ATA_MASK_* macros and reorder xfer_mask fields such
that higher transfer mode is placed at higher order bit.  As thie
reordering breaks ata_mode_string(), this patch also rewrites
ata_mode_string().

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