net: dsa: sja1105: implement the port MTU callbacks

On this switch, the frame length enforcements are performed by the
ingress policers. There are 2 types of those: regular L2 (also called
best-effort) and Virtual Link policers (an ARINC664/AFDX concept for
defining L2 streams with certain QoS abilities). To avoid future
confusion, I prefer to call the reset reason "Best-effort policers",
even though the VL policers are not yet supported.

We also need to change the setup of the initial static config, such that
DSA calls to .change_mtu (which are expensive) become no-ops and don't
reset the switch 5 times.

A driver-level decision is to unconditionally allow single VLAN-tagged
traffic on all ports. The CPU port must accept an additional VLAN header
for the DSA tag, which is again a driver-level decision.

The policers actually count bytes not only from the SDU, but also from
the Ethernet header and FCS, so those need to be accounted for as well.

Signed-off-by: Vladimir Oltean <[email protected]>
Reviewed-by: Florian Fainelli <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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