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>