net: bridge: br_set_ageing_time takes a clock_t

Change the ageing_time type in br_set_ageing_time() from u32 to what it
is expected to be, i.e. a clock_t.

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