ahci, msix: Fix build error for !PCI_MSI

It turned out the irq vector of the msix can be obtained from struct
msix_entry. This makes the lookup function for msi_desc obsolete.

This fixes a build error if PCI_MSI is unset:

 drivers/ata/ahci.c: In function ‘msix_get_desc’:
 drivers/ata/ahci.c:1210:2: error: ‘struct pci_dev’ has no member named ‘msi_list’

Catched by Fengguang's build bot.
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/ata/ahci.c

@@ -1203,18 +1203,6 @@ static inline void ahci_gtf_filter_workaround(struct ata_host *host)
 {}
 #endif
 
-static struct msi_desc *msix_get_desc(struct pci_dev *dev, u16 entry)
-{
-	struct msi_desc *desc;
-
-	list_for_each_entry(desc, &dev->msi_list, list) {
-		if (desc->msi_attrib.entry_nr == entry)
-			return desc;
-	}
-
-	return NULL;
-}
-
 /*
  * ahci_init_msix() only implements single MSI-X support, not multiple
  * MSI-X per-port interrupts. This is needed for host controllers that only
@@ -1223,7 +1211,6 @@ static struct msi_desc *msix_get_desc(struct pci_dev *dev, u16 entry)
 static int ahci_init_msix(struct pci_dev *pdev, unsigned int n_ports,
 			  struct ahci_host_priv *hpriv)
 {
-	struct msi_desc *desc;
 	int rc, nvec;
 	struct msix_entry entry = {};
 
@@ -1248,13 +1235,7 @@ static int ahci_init_msix(struct pci_dev *pdev, unsigned int n_ports,
 	if (rc < 0)
 		goto fail;
 
-	desc = msix_get_desc(pdev, 0);	/* first entry */
-	if (!desc) {
-		rc = -EINVAL;
-		goto fail;
-	}
-
-	hpriv->irq = desc->irq;
+	hpriv->irq = entry.vector;
 
 	return 1;
 fail: