net/smc: reduce port_event scheduling

IB event handlers schedule the port event worker for further
processing of port state changes. This patch reduces the number of
schedules to avoid duplicate processing of the same port change.

Reviewed-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>