[NET]: Let USB_USBNET always select MII.

All this USB_USBNET_MII trickery is simply not worth it considering how
few code it saves.

As a side effect, this also fixes the following compile error reported
by Toralf Frster:

<--  snip  -->

...
  LD      .tmp_vmlinux1
drivers/built-in.o: In function `usbnet_set_settings':
(.text+0xf1876): undefined reference to `mii_ethtool_sset'
drivers/built-in.o: In function `usbnet_get_settings':
(.text+0xf1836): undefined reference to `mii_ethtool_gset'
drivers/built-in.o: In function `usbnet_get_link':
(.text+0xf18d6): undefined reference to `mii_link_ok'
drivers/built-in.o: In function `usbnet_nway_reset':
(.text+0xf18f6): undefined reference to `mii_nway_restart'
make: *** [.tmp_vmlinux1] Error 1

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>