resolve collision of ATA_FLAG_LOWTAG and driver specific flag

The recently introduced ATA_FLAG_LOWTAG accidentially uses the same bit as a 
flag in sata_sil24. This causes problems with Silicon Image 3132.

As ATA_FLAG_LOWTAG is only used in sata_sil24 and is planned to be removed soon, 
so lets just use another bit for the flag in sata_sil24 for the stable series. 

Signed-off-by: Ronny Hegewald <Ronny.Hegewald@online.de>
Cc: stable@vger.kernel.org # 3.18 + 3.19

--- linux-3.18.5/drivers/ata/sata_sil24.c.org
+++ linux-3.18.5/drivers/ata/sata_sil24.c
@@ -247,7 +247,7 @@
 	SIL24_COMMON_FLAGS	= ATA_FLAG_SATA | ATA_FLAG_PIO_DMA |
 				  ATA_FLAG_NCQ | ATA_FLAG_ACPI_SATA |
 				  ATA_FLAG_AN | ATA_FLAG_PMP | ATA_FLAG_LOWTAG,
-	SIL24_FLAG_PCIX_IRQ_WOC	= (1 << 24), /* IRQ loss errata on PCI-X */
+	SIL24_FLAG_PCIX_IRQ_WOC	= (1 << 25), /* IRQ loss errata on PCI-X */
 
 	IRQ_STAT_4PORTS		= 0xf,
 };