gve: add support to change ring size via ethtool

Allow the user to change ring size via ethtool if
supported by the device. The driver relies on the
ring size ranges queried from device to validate
ring sizes requested by the user.

Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Harshitha Ramamurthy <hramamurthy@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>