Export the following xfermode related functions.

* ata_pack_xfermask()
* ata_unpack_xfermask()
* ata_xfer_mask2mode()
* ata_xfer_mode2mask()
* ata_xfer_mode2shift()
* ata_mode_string()
* ata_id_xfermask()
* ata_timing_find_mode()

These functions will be used later by LLD updates.  While at it,
change unsigned short @speed to u8 @xfer_mode in
ata_timing_find_mode() for consistency.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>