net: macb: check clk_set_rate return value to be negative

clk_set_rate() returns the set rate in case of success and a
negative number in case of failure. Consider failure only the
negative numbers.

Fixes: 3ef64444de ("dm: net: macb: Implement link speed change callback")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>

drivers/net/macb.c

@@ -631,7 +631,7 @@ int __weak macb_linkspd_cb(struct udevice *dev, unsigned int speed)
 
 	if (tx_clk.dev) {
 		ret = clk_set_rate(&tx_clk, rate);
-		if (ret)
+		if (ret < 0)
 			return ret;
 	}
 #endif