firmware_loader: document firmware_sysfs_fallback()

This also sets the expecations for future fallback interfaces, even
if they are not exported.

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>