mlxsw: spectrum: Fix ageing time value

The value passed through switchdev attr set is not in jiffies, but in
clock_t, so fix the convert.

Reported-by: Sagi Rotem <sagir@mellanox.com>
Fixes: 56ade8fe

 ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>