iflib drivers: Constify PCI ID LUTs

Since d49e83eac3baf16a22b1c5d42e8438b68b17e6f9, iflib(9) is ready
for this change.
While at it, make isc_driver_version strings (static) const where
not apparently un-const on purpose, too.
This reduces the size of the amd64 GENERIC by about 10 KiB.