[PATCH] libata: separate out ata_spd_string()

Separate out ata_spd_string() from sata_print_link_status().  This
will be used by SATA spd configuration routines.

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