sctp: Correclty set changeover_active for SFR-CACC

Right now, any time we set a primary transport we set
the changeover_active flag.  As a result, we invoke SFR-CACC
even when there has been no changeover events.

Only set changeover_active, when there is a true changeover
event, i.e. we had a primary path and we are changing to
another transport.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>