net: dsa: change scope of ageing time setter

Change the scope of the switchdev bridge ageing time attribute setter
from the DSA slave device to the generic DSA port, so that the future
port-wide API can also be used for other port types, such as CPU and DSA
links.

Also ds->ports is now a contiguous array of dsa_port structures, thus
their addresses cannot be NULL. Remove the useless check in
dsa_fastest_ageing_time.

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>