wimax: i2400m-usb - use a private struct ethtool_ops

This way the USB variant of the driver uses usb_make_path in order to
provide bus-info compatible to other USB drivers (like e.g. asix.c).

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>