net: phy: meson-gxl: Add generic dummy stubs for MMD register access

The Meson G12A Internal PHY does not support standard IEEE MMD extended
register access, therefore add generic dummy stubs to fail the read and
write MMD calls. This is necessary to prevent the core PHY code from
erroneously believing that EEE is supported by this PHY even though this
PHY does not support EEE, as MMD register access returns all FFFFs.

Fixes: 5c3407abb338 ("net: phy: meson-gxl: add g12a support")
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Chris Healy <healych@amazon.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://lore.kernel.org/r/20230130231402.471493-1-cphealy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/meson-gxl.c

@@ -271,6 +271,8 @@ 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,
 	},
 };