libata: fix probe time irq printouts

Most drivers don't seem to fill out the host->irq field, resulting in the
wrong (no) irq being reported at probe time. For example, sil24 on my system:

ata1: SATA max UDMA/100 cmd 0xd00008009001f000 ctl 0x0000000000000000 bmdma 0x0000000000000000 irq 0
ata2: SATA max UDMA/100 cmd 0xd000080090021000 ctl 0x0000000000000000 bmdma 0x0000000000000000 irq 0

Since they're allocated and set up in ata_host_activate(), just save
them away there.
Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -6435,6 +6435,9 @@ int ata_host_activate(struct ata_host *host, int irq,
 	if (rc)
 		devm_free_irq(host->dev, irq, host);
 
+	/* Used to print device info at probe */
+	host->irq = irq;
+
 	return rc;
 }