PCI/MSI: Retrieve first MSI IRQ from msi_desc rather than pci_dev

Retrieve the first MSI IRQ to compute the MSI index from struct msi_desc
rather than the struct pci_dev to avoid an additional memory access.
Signed-off-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/msi.c

@@ -235,7 +235,7 @@ static void msi_set_mask_bit(struct irq_data *data, u32 flag)
 		msix_mask_irq(desc, flag);
 		readl(desc->mask_base);		/* Flush write to device */
 	} else {
-		unsigned offset = data->irq - desc->dev->irq;
+		unsigned offset = data->irq - desc->irq;
 		msi_mask_irq(desc, 1 << offset, flag << offset);
 	}
 }