sctp: fix an error code in sctp_sf_eat_auth()

The sctp_sf_eat_auth() function is supposed to enum sctp_disposition
values and returning a kernel error code will cause issues in the
caller.  Change -ENOMEM to SCTP_DISPOSITION_NOMEM.

Fixes: 65b07e5d

 ("[SCTP]: API updates to suport SCTP-AUTH extensions.")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Acked-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>