net: ethernet: mtk_star_emac: fix unused variable

Fix the following warning in mtk_star_emac.c if CONFIG_OF is not set:

drivers/net/ethernet/mediatek/mtk_star_emac.c:1559:34:
    warning: unused variable 'mtk_star_of_match' [-Wunused-const-variable]
    static const struct of_device_id mtk_star_of_match[] = {

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>