Documentation: remove stale firmware API reference

It refers to a pending patch, but this was merged eons ago.

Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>