libata-sff: use WARN instead of BUG on illegal host state machine state

ata_sff_hsm_move() triggers BUG if it sees a host state machine state
that it dind't expect.  The risk for data corruption when the
condition occurs is low as it's highly unlikely that it would lead to
spurious completion of commands.  The BUG occasionally triggered for
subtle race conditions in the driver.  Let's downgrade it to WARN so
that it doesn't kill the machine unnecessarily.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>