net: dccp : proto: remove Unneeded variable "err"

Fix below issue reported by coccicheck

net/dccp/proto.c:266:5-8: Unneeded variable: "err". Return "0" on line
310

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>