netfilter: nf_ct_dccp/udplite: fix protocol registration error

Commit d0dba725 (netfilter: ctnetlink: add callbacks to the per-proto
nlattrs) changed the protocol registration function to abort if the
to-be registered protocol doesn't provide a new callback function.

The DCCP and UDP-Lite IPv6 protocols were missed in this conversion,
add the required callback pointer.

Reported-and-tested-by: Steven Jan Springl <steven@springl.ukfsn.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>