libata: make WARN_ON conditions in ata_sff_hsm_move() more strict

WARN_ON()'s in ata_hsm_move() was too liberal and got triggerred when
it shouldn't (e.g. hotplug events at the right moment).  As the HSM
only deals with device errors and state machine violations, make it
check only against them.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Mark Lord <liml@rtr.ca>
Cc: Albert Lee <albertcc@tw.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>