dsa: Move switch drivers to new directory drivers/net/dsa

Support for specific hardware belongs under drivers/net/ not net/.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>