sctp: fix the return value of getting the sctp partial delivery point

Get the sctp partial delivery point using SCTP_PARTIAL_DELIVERY_POINT
socket option should return 0 if success, not -ENOTSUPP.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>