net: dsa: dsa_fastest_ageing_time return unsigned

The ageing time is defined as unsigned int, so make
dsa_fastest_ageing_time return an unsigned int instead of int.

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>