mlxsw: ethtool: Pass link mode in use to ethtool

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

Instead, pass the link mode bit in use.
In Spectrum-1, simply pass the bit that is set to '1' from PTYS register.
In Spectrum-2, pass the first link mode bit in the mask of the used
link mode.

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