driver: net: ethernet: mtk_star_emac: fix suspend/resume issue

Identify the cause of the suspend/resume hang: netif_carrier_off()
is called during link state changes and becomes stuck while
executing linkwatch_work().

To resolve this issue, call netif_device_detach() during the Ethernet
suspend process to temporarily detach the network device from the
kernel and prevent the suspend/resume hang.

Fixes: 8c7bd5a454ff ("net: ethernet: mtk-star-emac: new driver")
Signed-off-by: Yanqing Wang <[e-mail]>
Signed-off-by: Macpaul Lin <[e-mail]>
Signed-off-by: Biao Huang <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/ethernet/mediatek/mtk_star_emac.c

···
 	if (netif_running(ndev))
 		mtk_star_disable(ndev);
 
+	netif_device_detach(ndev);
+
 	clk_bulk_disable_unprepare(MTK_STAR_NCLKS, priv->clks);
 
 	return 0;
···
 		if (ret)
 			clk_bulk_disable_unprepare(MTK_STAR_NCLKS, priv->clks);
 	}
+
+	netif_device_attach(ndev);
 
 	return ret;
 }