dsa: Remove master_dev from switch structure

The switch drivers only use the master_dev member for dev_info()
messages.  Now that the device is passed to the old style probe, and
new style drivers are probed as true linux drivers, this is no longer
needed.

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