net: phy: meson-gxl: use MMD access dummy stubs for GXL, internal PHY

Jerome provided the information that also the GXL internal PHY doesn't
support MMD register access and EEE. MMD reads return 0xffff, what
results in e.g. completely wrong ethtool --show-eee output.
Therefore use the MMD dummy stubs.

Fixes: d853d14 ("net: phy: add an option to disable EEE advertisement")
Suggested-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/84432fe4-0be4-bc82-4e5c-557206b40f56@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/meson-gxl.c

@@ -261,6 +261,8 @@ static struct phy_driver meson_gxl_phy[] = {
 		.handle_interrupt = meson_gxl_handle_interrupt,
 		.suspend        = genphy_suspend,
 		.resume         = genphy_resume,
+		.read_mmd	= genphy_read_mmd_unsupported,
+		.write_mmd	= genphy_write_mmd_unsupported,
 	}, {
 		PHY_ID_MATCH_EXACT(0x01803301),
 		.name		= "Meson G12A Internal PHY",