selftests: mlxsw: Reduce running time using offload indication

After adding a given number of flower rules for different IPv6
addresses, the test generates traffic and ensures that each packet is
received, which is time-consuming.

Instead, test the offload indication of the tc flower rules and reduce
the running time by half.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>