net: HNS: fix MDIO dependencies

The newly introduced HNS_MDIO Kconfig symbol selects 'MDIO', but
that is the wrong symbol as the code used by this driver is
provided by PHYLIB rather than the MDIO driver. Also, there is
no need to make this driver user selectable, because it is already
selected by all drivers that need it.

This changes the Kconfig file to select the correct library, and
to make the option silent.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5b904d39

 ("net: add Hisilicon Network Subsystem MDIO support")
Signed-off-by: David S. Miller <davem@davemloft.net>