gnrc_ipv6_nib: use router instead of netif for router timeout context
gnrc_ipv6_nib: use router instead of netif for router timeout context

`GNRC_IPV6_NIB_RTR_TIMEOUT` [expects a default router entry][doc] as
context, so using the `netif` here is clearly wrong. This causes the
hardfault described in #9297.

[doc]: http://doc.riot-os.org/group__net__gnrc__ipv6__nib.html#ga1c7e892ef6533f699c4e64737c2847c2

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