mlxsw: spectrum: Seed LAG hash function

Currently, the seed of the LAG hash function is always set to 0, which
means it is identical across all switches. Instead, use a random number.

This is especially important now that VxLAN is supported, as the LAG
hash function is used to calculate the UDP source port of the
encapsulated packet.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>