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>
diff --git a/drivers/net/ethernet/3com/Kconfig b/drivers/net/ethernet/3com/Kconfig
index a439cbd..a8bb30c 100644
--- a/drivers/net/ethernet/3com/Kconfig
+++ b/drivers/net/ethernet/3com/Kconfig
@@ -81,6 +81,7 @@
 config VORTEX
 	tristate "3c590/3c900 series (592/595/597) \"Vortex/Boomerang\" support"
 	depends on (PCI || EISA)
+	select NET_CORE
 	select MII
 	---help---
 	  This option enables driver support for a large number of 10Mbps and