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>