[libata] fix oops on non-DMA bmdma hardware

Alan noted: "bmdma may be zero but the bmdma_irq_clear function gets
called even in this case during pure PIO operation. Check we have a
bmdma before we use it."

I fixed this by adding a check for zero.  While was I there, I fixed the
non-standard indentation of the small function's code.
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/scsi/libata-core.c

@@ -4202,14 +4202,17 @@ void ata_bmdma_setup(struct ata_queued_cmd *qc)
 
 void ata_bmdma_irq_clear(struct ata_port *ap)
 {
-    if (ap->flags & ATA_FLAG_MMIO) {
-        void __iomem *mmio = ((void __iomem *) ap->ioaddr.bmdma_addr) + ATA_DMA_STATUS;
-        writeb(readb(mmio), mmio);
-    } else {
-        unsigned long addr = ap->ioaddr.bmdma_addr + ATA_DMA_STATUS;
-        outb(inb(addr), addr);
-    }
+	if (!ap->ioaddr.bmdma_addr)
+		return;
 
+	if (ap->flags & ATA_FLAG_MMIO) {
+		void __iomem *mmio =
+		      ((void __iomem *) ap->ioaddr.bmdma_addr) + ATA_DMA_STATUS;
+		writeb(readb(mmio), mmio);
+	} else {
+		unsigned long addr = ap->ioaddr.bmdma_addr + ATA_DMA_STATUS;
+		outb(inb(addr), addr);
+	}
 }