[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>

drivers/ata/libata-sff.c

@@ -1333,6 +1333,7 @@ void ata_sff_flush_pio_task(struct ata_port *ap)
 
 	cancel_delayed_work_sync(&ap->sff_pio_task);
 	ap->hsm_task_state = HSM_ST_IDLE;
+	ap->sff_pio_task_link = NULL;
 
 	if (ata_msg_ctl(ap))
 		ata_port_dbg(ap, "%s: EXIT\n", __func__);