MII: fix Kconfig dependencies for MII

MII Kconfig option is apart of the core networking drivers and
by default NET_CORE is enabled so drivers selecting MII will
have MII enabled as well.  It was found using the randconfig
option during testing, MII would be selected but NET_CORE
could be disabled.  This caused a dependency error.

Resolved the dependency by selecting NET_CORE when MII is
selected.

Reported-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/nuvoton/Kconfig

@@ -22,6 +22,7 @@ config W90P910_ETH
 	tristate "Nuvoton w90p910 Ethernet support"
 	depends on ARM && ARCH_W90X900
 	select PHYLIB
+	select NET_CORE
 	select MII
 	---help---
 	  Say Y here if you want to use built-in Ethernet ports