net: stmmac: Fallback to dev_fwnode() if needed

When CONFIG_OF is not enabled, of_fwnode_handle() will return NULL, even
though we can have a FW handle from a given device.

Fallback to dev_fwnode() helper if needed.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>