ath79: ag71xx: unify version dependent code

Use IS_ERR_OR_NULL macro to use the same code on kernel 4.19 as well as
5.4.

Signed-off-by: David Bauer <[email protected]>

target/linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -1549,11 +1549,7 @@ static int ag71xx_probe(struct platform_device *pdev)
 	ag->stop_desc->next = (u32) ag->stop_desc_dma;
 
 	mac_addr = of_get_mac_address(np);
-#if (LINUX_VERSION_CODE < KERNEL_VERSION(5,2,0))
-	if (!mac_addr || !is_valid_ether_addr(mac_addr)) {
-#else
-	if (IS_ERR(mac_addr) || !is_valid_ether_addr(mac_addr)) {
-#endif
+	if (IS_ERR_OR_NULL(mac_addr) || !is_valid_ether_addr(mac_addr)) {
 		dev_err(&pdev->dev, "invalid MAC address, using random address\n");
 		eth_random_addr(dev->dev_addr);
 	} else {