LinuxKPI: implement irq_get_msi_desc()

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.

MFC after: 3 days
Reviewed by: hselasky (earlier version)
Differential Revision: https://reviews.freebsd.org/D37523

(cherry picked from commit 4b56afaf7bf4fa37bae5b26fd93ee1ff5969c1bb)