net: stmmac: drop the reset delays from struct stmmac_mdio_bus_data

Only OF platforms use the reset delays and these delays are only read in
stmmac_mdio_reset(). Move them from struct stmmac_mdio_bus_data to a
stack variable inside stmmac_mdio_reset() because that's the only usage
of these delays.

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