ethtool: Factored out similar ethtool link settings for virtual devices to core

Three virtual devices (ibmveth, virtio_net, and netvsc) all have
similar code to set link settings and validate ethtool command. To
eliminate duplication of code, it is factored out into core/ethtool.c.

Signed-off-by: Cris Forno <cforno12@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>