net: dsa: mv88e6xxx: Decode ATU problem interrupt

When there is a problem with the ATU, an interrupt can be
generated. Trap this interrupt and decode the registers to determine
what the problem was, then log the error.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>