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>