[libata] PIO error handling improvement

Tested burning CD-RW with libata-dev-2.6 and cdrecord:
1. ATAPI DMA mode - tested OK
2. ATAPI PIO mode - test failed when cdrecord finishes burning and issues MODE_SELECT to the device.

 After checking the log, it showed that MODE_SELECT caused ata_pio_complete() to return error.
However, the error is not handled by ata_pio_task().

Attached please find the patch for ata_pio_task() error handling for your review.
(The patch is against the libata-dev-2.6 tree. )

Changes in the patch:
1. End the PIO task when PIO_ST_IDLE state is entered
2. End the PIO task after PIO_ST_TMOUT and PIO_ST_ERR state handled by ata_pio_error()
3. Remove the first "if" statement to handle the error condition returned from 
   ata_pio_block(), ata_pio_complete() and ata_pio_poll().

Change #2 is not so necessary since ata_pio_error() will put the cmd to  PIO_ST_IDLE state
after the error condition is handled. The change just saves a function call to queue_work().

Tested OK on on my machine with pdc20275 and ASUS CD-RW drive.
Signed-off-by: Albert Lee <albertcc@tw.ibm.com>

drivers/scsi/libata-core.c

@@ -2369,6 +2369,9 @@ static void ata_pio_task(void *_data)
 	unsigned long timeout = 0;
 
 	switch (ap->pio_task_state) {
+	case PIO_ST_IDLE:
+		return;
+
 	case PIO_ST:
 		ata_pio_block(ap);
 		break;
@@ -2385,18 +2388,14 @@ static void ata_pio_task(void *_data)
 	case PIO_ST_TMOUT:
 	case PIO_ST_ERR:
 		ata_pio_error(ap);
-		break;
+		return;
 	}
 
-	if ((ap->pio_task_state != PIO_ST_IDLE) &&
-	    (ap->pio_task_state != PIO_ST_TMOUT) &&
-	    (ap->pio_task_state != PIO_ST_ERR)) {
 	if (timeout)
 		queue_delayed_work(ata_wq, &ap->pio_task,
 				   timeout);
 	else
 		queue_work(ata_wq, &ap->pio_task);
-	}
 }
 
 static void atapi_request_sense(struct ata_port *ap, struct ata_device *dev,