constify struct cfattach

Constify struct {audio,midi,radio,video}_hw_if. No functional change.

ok mpi@ ratchov@ "More const is good" deraadt@

Add escodec(4), a driver for the Everest ES8316 audio codec,
which is used on the Pinebook Pro.

Driver written by Jared McNeill at NetBSD
ok kettenis@