net: ftgmac100: remove empty MDIO bus reset function

ftgmac100_mdiobus_reset() does nothing useful and is optional for the
MDIO bus code, so let's just remove it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>