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.

(cherry picked from commit 51e235148a4becba94e824a44bd69687644a7f56)

Conflicts (drivers missing in stable/13):
sys/dev/enetc/if_enetc.c
sys/dev/enic/if_enic.c