netfilter: decnet: use NFPROTO values for NF_HOOK invocation

The semantic patch used was:
// <smpl>
@@
@@
 NF_HOOK(
-PF_DECnet,
+NFPROTO_DECNET,
 ...)
// </smpl>

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>