sctp: add the missing setting for asoc encap_port

This patch is to add the missing setting back for asoc encap_port.

Fixes: 8dba2960

 ("sctp: add SCTP_REMOTE_UDP_ENCAPS_PORT sockopt")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>