net: dsa: mv88e6xxx: debug ATU Age Time

The ATU ageing time value programmed in the switch is rounded up to the
nearest multiple of its coefficient (variable depending on the model.)

Add a debug message to inform the user about the exact programmed value.

On 6352, "brctl setageing br0 18" gives "AgeTime set to 0x01 (15000 ms)"
while on 6390 we get "AgeTime set to 0x05 (18750 ms)".

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>