net: Move MII out from under NET_CORE and hide it

All drivers that select MII also need to select NET_CORE because MII
depends on it.  This is a bit ridiculous because NET_CORE is just a
menu option that doesn't enable any code by itself.

There is also no need for it to be a visible option, since its users
all select it.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/rdc/Kconfig

@@ -22,7 +22,6 @@ config R6040
 	tristate "RDC R6040 Fast Ethernet Adapter support"
 	depends on PCI
 	select CRC32
-	select NET_CORE
 	select MII
 	select PHYLIB
 	---help---