net: dsa: lantiq_gswip: mark OF related data as maybe unused

The driver can be compile tested with !CONFIG_OF making certain data
unused:

  drivers/net/dsa/lantiq_gswip.c:1888:34: error: ‘xway_gphy_match’ defined but not used [-Werror=unused-const-variable=]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/lantiq_gswip.c

@@ -1885,7 +1885,7 @@ static const struct xway_gphy_match_data xrx300_gphy_data = {
 	.ge_firmware_name = "lantiq/xrx300_phy11g_a21.bin",
 };
 
-static const struct of_device_id xway_gphy_match[] = {
+static const struct of_device_id xway_gphy_match[] __maybe_unused = {
 	{ .compatible = "lantiq,xrx200-gphy-fw", .data = NULL },
 	{ .compatible = "lantiq,xrx200a1x-gphy-fw", .data = &xrx200a1x_gphy_data },
 	{ .compatible = "lantiq,xrx200a2x-gphy-fw", .data = &xrx200a2x_gphy_data },