[libata] Prevent warning during PMP error recovery

Cleanup sff_pio_task_link when a command is cancel while the
pio_task thread has been scheduled.

Signed-off-by: Gwendal Grignou <gwendal@google.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>