net/ethtool: Introduce link_ksettings API for virtual network devices

With the ethtool_virtdev_set_link_ksettings function in core/ethtool.c,
ibmveth, netvsc, and virtio now use the core's helper function.

Funtionality changes that pertain to ibmveth driver include:

  1. Changed the initial hardcoded link speed to 1GB.

  2. Added support for allowing a user to change the reported link
  speed via ethtool.

Functionality changes to the netvsc driver include:

  1. When netvsc_get_link_ksettings is called, it will defer to the VF
  device if it exists to pull accelerated networking values, otherwise
  pull default or user-defined values.

  2. Similarly, if netvsc_set_link_ksettings called and a VF device
  exists, the real values of speed and duplex are changed.

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