sis5513: fix PIO setup for ATAPI devices

Clear prefetch setting before potentially (re-)enabling it in
config_drive_art_rwp() so the transition of the device type on
the port from ATA to ATAPI (i.e. during warm-plug operation)
is handled correctly.

This is a really old bug (it probably goes back to very early
days of the driver) but it was only affecting warm-plug operation
until the recent "ide: try to use PIO Mode 0 during probe if
possible" change (commit 6029336426).

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Tested-by: David Fries <david@fries.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/ide/sis5513.c

@@ -2,7 +2,7 @@
  * Copyright (C) 1999-2000	Andre Hedrick <andre@linux-ide.org>
  * Copyright (C) 2002		Lionel Bouton <Lionel.Bouton@inet6.fr>, Maintainer
  * Copyright (C) 2003		Vojtech Pavlik <vojtech@suse.cz>
- * Copyright (C) 2007		Bartlomiej Zolnierkiewicz
+ * Copyright (C) 2007-2009	Bartlomiej Zolnierkiewicz
  *
  * May be copied or modified under the terms of the GNU General Public License
  *
@@ -281,11 +281,13 @@ static void config_drive_art_rwp(ide_drive_t *drive)
 
 	pci_read_config_byte(dev, 0x4b, &reg4bh);
 
-	if (drive->media == ide_disk)
-		rw_prefetch = 0x11 << drive->dn;
+	rw_prefetch = reg4bh & ~(0x11 << drive->dn);
 
-	if ((reg4bh & (0x11 << drive->dn)) != rw_prefetch)
-		pci_write_config_byte(dev, 0x4b, reg4bh|rw_prefetch);
+	if (drive->media == ide_disk)
+		rw_prefetch |= 0x11 << drive->dn;
+
+	if (reg4bh != rw_prefetch)
+		pci_write_config_byte(dev, 0x4b, rw_prefetch);
 }
 
 static void sis_set_pio_mode(ide_drive_t *drive, const u8 pio)