net: stmmac: use "mdio_bus_data" local variable

We have a local variable for priv->plat->mdio_bus_data, which we use
later in the conditional if() block, but we evaluate the above within
the conditional expression. Use mdio_bus_data instead. Since these
will be the only two users of this local variable, move its assignment
just before the if().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qZAXJ-005pU1-1z@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>