net: stmmac: Fix stmmac_mdio_reset() when building stmmac as modules

When building stmmac, it is only possible to select CONFIG_DWMAC_GENERIC,
or any of the glue drivers, when CONFIG_STMMAC_PLATFORM is set.
The only exception is CONFIG_STMMAC_PCI.

When calling of_mdiobus_register(), it will call our ->reset()
callback, which is set to stmmac_mdio_reset().

Most of the code in stmmac_mdio_reset() is protected by a
"#if defined(CONFIG_STMMAC_PLATFORM)", which will evaluate
to false when CONFIG_STMMAC_PLATFORM=m.

Because of this, the phy reset gpio will only be pulled when
stmmac is built as built-in, but not when built as modules.

Fix this by using "#if IS_ENABLED()" instead of "#if defined()".

Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>