net: dsa: vsc73xx: fix NET_DSA and OF dependencies

The restructuring of the driver got the dependencies wrong: without
CONFIG_NET_DSA we get this build failure:

WARNING: unmet direct dependencies detected for NET_DSA_VITESSE_VSC73XX
  Depends on [n]: NETDEVICES [=y] && HAVE_NET_DSA [=y] && OF [=y] && NET_DSA [=n]
  Selected by [m]:
  - NET_DSA_VITESSE_VSC73XX_PLATFORM [=m] && NETDEVICES [=y] && HAVE_NET_DSA [=y] && HAS_IOMEM [=y]

ERROR: "dsa_unregister_switch" [drivers/net/dsa/vitesse-vsc73xx-core.ko] undefined!
ERROR: "dsa_switch_alloc" [drivers/net/dsa/vitesse-vsc73xx-core.ko] undefined!
ERROR: "dsa_register_switch" [drivers/net/dsa/vitesse-vsc73xx-core.ko] undefined!

Add the appropriate dependencies.

Fixes: 95711cd5 ("net: dsa: vsc73xx: Split vsc73xx driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>