net: dsa: add notifier for ageing time

This patch keeps the port-wide ageing time handling code in
dsa_port_ageing_time, pushes the requested ageing time value in a new
switch fabric notification, and moves the switch-wide ageing time
handling code in dsa_switch_ageing_time.

This has the effect that now not only the switch that the target port
belongs to can be programmed, but all switches composing the switch
fabric. For the moment, keep the current behavior and ignore other
switches.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>