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>