ethernet: eth_stm32_hal: fix unused variable build warning

Fix an unused variable build warning that was happening in certain
configurations. Move the variables in the only condition where they are
actually used.

west build -p -b nucleo_f429zi \
	-T samples/net/cloud/aws_iot_mqtt/sample.net.cloud.aws_iot_mqtt

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>