um: Don't overwrite ethtool driver version

In-tree drivers don't need to manage internal version because
they are aligned to the global Linux kernel version, which is
reported by default with "ethtool -i".

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>