selftests: mlxsw: Set headroom size of correct port

The test was setting the headroom size of the wrong port. This was not
visible because of a firmware bug that canceled this bug.

Set the headroom size of the correct port, so that the test will pass
with both old and new firmware versions.

Fixes: bfa8047 ("selftests: mlxsw: Add a PFC test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20201230114251.394009-1-idosch@idosch.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: idosch

tools/testing/selftests/drivers/net/mlxsw/qos_pfc.sh

@@ -230,7 +230,7 @@ switch_create()
 	__mlnx_qos -i $swp4 --pfc=0,1,0,0,0,0,0,0 >/dev/null
 	# PG0 will get autoconfigured to Xoff, give PG1 arbitrarily 100K, which
 	# is (-2*MTU) about 80K of delay provision.
-	__mlnx_qos -i $swp3 --buffer_size=0,$_100KB,0,0,0,0,0,0 >/dev/null
+	__mlnx_qos -i $swp4 --buffer_size=0,$_100KB,0,0,0,0,0,0 >/dev/null
 
 	# bridges
 	# -------