mlxsw: PCI: Send EMAD traffic on a separate queue

Currently mlxsw distributes sent traffic among all the available send
queues. That includes control traffic as well as EMADs, which are used for
configuration of the device.

However because all the queues have the same traffic class of 3, they all
end up being directed to the same traffic class buffer. If the control
traffic in the buffer cannot be serviced quickly enough, the EMAD traffic
might be shut out, which causes transient failures, typically in FDB
maintenance, counter upkeep and other periodic work.

To address this issue, dedicate SDQ 0 to EMAD traffic, with TC 0.
Distribute the control traffic among the remaining queues, which are left
with their current TC 3.

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