sctp: sm_statefuns: Remove pointer casts of the same type

The subh.addip_hdr pointer is also of type (struct sctp_addiphdr *), so
it does not require a cast.

Signed-off-by: Li zeming <zeming@nfschina.com>
Link: https://lore.kernel.org/r/20221115020705.3220-1-zeming@nfschina.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>