realtek: re-enable IPv6 routing

realtek: re-enable IPv6 routing

Commit 03e1d93e0779 ("realtek: add driver support for routing
offload") added routing offload for IPv4, but broke IPv6 routing
completely.  The routing table is empty and cannot be updated:

 root@gs1900-10hp:~# ip -6 route
 root@gs1900-10hp:~# ip -6 route add unreachable default
 RTNETLINK answers: Invalid argument

As a side effect, this breaks opkg on IPv4 only systems too,
since uclient-fetch fails when there are no IPv6 routes:

 root@gs1900-10hp:~# uclient-fetch http://192.168.99.1
 Downloading 'http://192.168.99.1'
 Failed to send request: Operation not permitted

Fix by returning NOTIFY_DONE when offloading is unsupported, falling
back to default behaviour.

Fixes: 03e1d93e0779 ("realtek: add driver support for routing offload")
Signed-off-by: Bjørn Mork <bjorn@mork.no>