net: uli526x: Fix unknown storage size error

The variable netdev_ethtool_ops is not referenced, drop it.  However
with gcc-6 or later we fail to even compile as we do not have the
required struct definition in U-Boot.

Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Tom Rini <trini@konsulko.com>