libata: check onlineness before using SPD in sata_down_spd_limit()

sata_down_spd_limit() should check whether the link is online before
using the SPD value to determine how to limit the link speed.  Factor
out onlineness test and test it from sata_down_spd_limit().

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>