net: dsa: Move mv88e6172 support into mv88e6352 family driver

The mv88e6172 is part of the mv88e6352 family of devices. Move support
for it out of the mv88e6171 driver into the mv88e6352, which results
in some simplifications to the code.

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