net: write enetaddr down to hardware on env_callback

If mac-address is changed using "setenv ethaddr ...." command the new
mac-adress also must be written into the responsible ethernet driver.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>