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/usb/Kconfig b/drivers/net/usb/Kconfig
index 84d4608..2335761 100644
--- a/drivers/net/usb/Kconfig
+++ b/drivers/net/usb/Kconfig
@@ -68,6 +68,7 @@
 
 config USB_PEGASUS
 	tristate "USB Pegasus/Pegasus-II based ethernet device support"
+	select NET_CORE
 	select MII
 	---help---
 	  Say Y here if you know you have Pegasus or Pegasus-II based adapter.
@@ -84,6 +85,7 @@
 config USB_RTL8150
 	tristate "USB RTL8150 based ethernet device support (EXPERIMENTAL)"
 	depends on EXPERIMENTAL
+	select NET_CORE
 	select MII
 	help
 	  Say Y here if you have RTL8150 based usb-ethernet adapter.
@@ -95,6 +97,7 @@
 
 config USB_USBNET
 	tristate "Multi-purpose USB Networking Framework"
+	select NET_CORE
 	select MII
 	---help---
 	  This driver supports several kinds of network links over USB,