selftests: mlxsw: qos_mc_aware: Configure shared buffers

This test runs two streams of traffic from two independent ports to
create congestion on one egress port. It is necessary to configure the
shared buffer thresholds correctly, to make sure that there is traffic
from both streams in the shared buffer. Only then can the test actually
test prioritization among these streams.

Without this configuration, it is possible, that one of the streams
takes all of port-pool quota, and the other stream is not even admitted,
thus invalidating the result.

On Spectrum-1, this is not a problem, because MC traffic uses a separate
pool. But for Spectrum-2, MC and UC share the same pool, and the correct
configuration is important.

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