net: don't override ethernet address environment

net: don't override ethernet address environment

If the ethernet address environment is set with a valid
ethernet address prevent overriding it as it is most likely
set by the user and he/she doesn't want board code to
automatically override it whatsoever.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>