sctp: avoid overwrite the return value of sctp_process_asconf_ack()

The return value of sctp_process_asconf_ack() may be
overwritten while process parameters with no error.
This patch fixed the problem.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>