net: stmmac: fix unused-variable warning

When building without CONFIG_OF, we get a harmless build warning:

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c: In function 'stmmac_phy_setup':
drivers/net/ethernet/stmicro/stmmac/stmmac_main.c:973:22: error: unused variable 'node' [-Werror=unused-variable]
  struct device_node *node = priv->plat->phy_node;

Reword it so we always use the local variable, by making it the
fwnode pointer instead of the device_node.

Fixes: 74371272 ("net: stmmac: Convert to phylink and remove phylib logic")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>