net: stmicro: fix a missing check of clk_prepare

clk_prepare() could fail, so let's check its status, and if it fails,
return its error code upstream.

Signed-off-by: Kangjie Lu <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/stmicro/stmmac/dwmac-sunxi.c

···
 		gmac->clk_enabled = 1;
 	} else {
 		clk_set_rate(gmac->tx_clk, SUN7I_GMAC_MII_RATE);
-		clk_prepare(gmac->tx_clk);
+		ret = clk_prepare(gmac->tx_clk);
+		if (ret)
+			return ret;
 	}
 
 	return 0;