ethtool: Get link mode in use instead of speed and duplex parameters

Currently, when user space queries the link's parameters, as speed and
duplex, each parameter is passed from the driver to ethtool.

Instead, get the link mode bit in use, and derive each of the parameters
from it in ethtool.

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>