firewire: Add deprecation notice

This was originally discussed as part of FreeBSD 15 planning, but did
not happen in time. Add the deprecation notice now, with an expectation
that it will be removed before FreeBSD 16.

Reviewed by: brooks, imp
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48598