r8169: select MII in Kconfig

drivers/built-in.o: In function `rtl8169_gset_xmii':
r8169.c:(.text+0x82259): undefined reference to `mii_ethtool_gset'

Signed-off-by: Hugh Dickins <hugh@veritas.com>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>