ipmi: Add or fix SPDX-License-Identifier in all files

And get rid of the license text that is no longer necessary.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Alistair Popple <alistair@popple.id.au>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Rocky Craig <rocky.craig@hp.com>