net: ethernet: mtk_eth_soc: fix error checking in mtk_mac_config()

Clang static analysis reports this problem
mtk_eth_soc.c:394:7: warning: Branch condition evaluates
  to a garbage value
                if (err)
                    ^~~

err is not initialized and only conditionally set.
So intitialize err.

Fixes: 7e538372 ("net: ethernet: mediatek: Re-add support SGMII")
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mediatek/mtk_eth_soc.c

@@ -267,7 +267,7 @@ static void mtk_mac_config(struct phylink_config *config, unsigned int mode,
 					   phylink_config);
 	struct mtk_eth *eth = mac->hw;
 	u32 mcr_cur, mcr_new, sid, i;
-	int val, ge_mode, err;
+	int val, ge_mode, err = 0;
 
 	/* MT76x8 has no hardware settings between for the MAC */
 	if (!MTK_HAS_CAPS(eth->soc->caps, MTK_SOC_MT7628) &&