net: dsa: mv88e6xxx: Preserve priority when setting CPU port.

The 6390 family uses an extended register to set the port connected to
the CPU. The lower 5 bits indicate the port, the upper three bits are
the priority of the frames as they pass through the switch, what
egress queue they should use, etc. Since frames being set to the CPU
are typically management frames, BPDU, IGMP, ARP, etc set the priority
to 7, the reset default, and the highest.

Fixes: 33641994

 ("net: dsa: mv88e6xxx: Monitor and Management tables")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>