firmware: move completing fw into a helper

This will be re-used later through a new extensible interface.

Reviewed-by: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>