LinuxKPI: pci: add more functions

Add a dummy pci_assign_resource() and an implementation of
pci_irq_vector() returning the irq for MSI-X, MSI, and legacy interrupt.
Both are needed by wirless drivers.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D38237