libata-sff: kill spurious WARN_ON() in ata_hsm_move()

On HSM_ST_ERR, ata_hsm_move() triggers WARN_ON() if AC_ERR_DEV or
AC_ERR_HSM is not set.  PHY events may trigger HSM_ST_ERR with other
error codes and, with or without it, there just isn't much reason to
do WARN_ON() on it.  Even if error code is not set there, core EH
logic won't have any problem dealing with the error condition.

OSDL bz#11065 reports this problem.

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