[][kernel][common][eth][Fix Coverity scan warning]

[Description]
Fix Coverity scan warning for the uninitialized variable in the
mtk_check_gmac23_idle().

[Release-log]
N/A


Change-Id: I1112546c2803a163cc7f50d272d89631c00fa496
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7487413
diff --git a/target/linux/mediatek/files-5.4/drivers/net/ethernet/mediatek/mtk_eth_soc.c b/target/linux/mediatek/files-5.4/drivers/net/ethernet/mediatek/mtk_eth_soc.c
index 686ddda..6f17fa5 100755
--- a/target/linux/mediatek/files-5.4/drivers/net/ethernet/mediatek/mtk_eth_soc.c
+++ b/target/linux/mediatek/files-5.4/drivers/net/ethernet/mediatek/mtk_eth_soc.c
@@ -474,6 +474,8 @@
 	case MTK_GMAC3_ID:
 		gdm_fsm = mtk_r32(mac->hw, MTK_FE_GDM3_FSM);
 		break;
+	default:
+		return true;
 	};
 
 	if ((mac_fsm & 0xFFFF0000) == 0x01010000 &&