kbd: replace vestigial spl calls with Giant assertions

The keyboard driver was initially protected via spl* interrupt priority
calls but (as part of a comprehensive effort) migrated to use the Giant
lock (mutex).

The spl calls left behind became NOPs but they can be confusing as they
have no bearing on the actual mutual exclusion that is now present.

Remove them from kbd and add assertions that Giant is held. markj notes
that there is conflation between the "bus topo" lock (which is Giant
under the hood) and Giant. The assertions could either be addressed as
a small item along with bus topology locking work or they'll be removed
if kbd is decoupled from Giant.

PR: 206680
Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34645