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: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>