firmware: add sanity check on shutdown/suspend

The firmware API should not be used after we go to suspend
and after we reboot/halt. The suspend/resume case is a bit
complex, so this documents that so things are clearer.

We want to know about users of the API in incorrect places so
that their callers are corrected, so this also adds a warn
for those cases.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>