When an ATU violation occurs, the switch uses the ATU FID register to
report the FID of the MAC address that incurred the violation. It would
be good for the driver to know the FID value for purposes such as
logging and CPU-based authentication.

Up until now, the driver has been calling the mv88e6xxx_g1_atu_op()
function to read ATU violations, but that doesn't do exactly what we
want, namely it calls mv88e6xxx_g1_atu_fid_write() with FID 0.
(side note, the documentation for the ATU Get/Clear Violation command
says that writes to the ATU FID register have no effect before the
operation starts, it's only that we disregard the value that this
register provides once the operation completes)

So mv88e6xxx_g1_atu_fid_write() is not what we want, but rather
mv88e6xxx_g1_atu_fid_read(). However, the latter doesn't exist, we need
to write it.

The remainder of mv88e6xxx_g1_atu_op() except for
mv88e6xxx_g1_atu_fid_write() is still needed, namely to send a
GET_CLR_VIOLATION command to the ATU. In principle we could have still
kept calling mv88e6xxx_g1_atu_op(), but the MDIO writes to the ATU FID
register are pointless, but in the interest of doing less CPU work per
interrupt, write a new function called mv88e6xxx_g1_read_atu_violation()
and call it.

The FID will be the port default FID as set by mv88e6xxx_port_set_fid()
if the VID from the packet cannot be found in the VTU. Otherwise it is
the FID derived from the VTU entry associated with that VID.
Signed-off-by: Hans J. Schultz <netdev@kapio-technology.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>