Add irq_get_msi_desc() as a wrapper around a PCI function which will
allocate a single cached value (see comment on struct) for the
msi_desc requested if it doesn't exist yet and handle freeing it
when the PCI device goes away. We take the values from the ivars of
the native (FreeBSD) device.

While changing struct pci_dev also add the msi_cap field requested by
a wireless driver.

Bump __FreeBSD_version so these changes can be detected.

MFC after: 3 days
X-MFC: move fields to end of struct (alloc happens in linux_pci.c)