sctp: no need to check assoc id before calling sctp_assoc_set_id

sctp_assoc_set_id does the assoc id check in the beginning when
processing dupcookie, no need to do the same check before calling
it.

v1->v2:
  fix some typo errs Marcelo pointed in changelog.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>