Correct call to eth_write_hwaddr()

This fixes "Warning: failed to set MAC address" on platforms which rely on
an 'ethaddr' environment variable to set the MAC address.

This bug was introduced by this commit:

7616e785 Add Ethernet hardware MAC address framework to usbnet

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kumar Gala <galak@kernel.crashing.org>
Tested-by: Michal Simek <monstr@monstr.eu>
Tested-by: Heiko Schocher <hs@denx.de>

net/eth.c

@@ -304,7 +304,7 @@ int eth_initialize(bd_t *bis)
 			if (strchr(dev->name, ' '))
 				puts("\nWarning: eth device name has a space!\n");
 
-			if (eth_write_hwaddr(dev, NULL, eth_number))
+			if (eth_write_hwaddr(dev, "eth", eth_number))
 				puts("Warning: failed to set MAC address\n");
 
 			eth_number++;