net: stmmac: drop the phy_reset hook from struct stmmac_mdio_bus_data

The phy_reset hook is not set anywhere. Drop it to make
stmmac_mdio_reset() smaller.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>