firmware: wrap FW_OPT_* into an enum

This should let us associate enum kdoc to these values.
While at it, kdocify the fw_opt.

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Luis R. Rodriguez <mcgrof@kernel.org>
[mcgrof: coding style fixes, merge kdoc with enum move]
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>