device: set `accept_ra` to 1 when changing IPv6 kernel token

Setting the kernel token is not strictly necessary as the IPv6 address is generated in userspace by NetworkManager. However it is convenient for users to see that the value set in the profile is also set in the kernel, to confirm that everything is working as expected.

The kernel allows setting a token only when 'accept_ra' is 1: temporarily flip it if necessary. Unfortunately this will also generate an additional Router Solicitation from kernel, but this is not a big issue.